I see so many questions on installing S-10 T-5 transmissions into old trucks I thought I'd write down my experience and offer it as tech reference. Instructions are for installing in a Advance Design model truck (47 to 53) but I think would be very similar for other years.
The tricky parts of this swap are putting together a clutch package and finding the right ratio's tranny for your purpose. These trannys don't have a great reputation with muscle car enthusiasts who run them with V-8's, but they seem to be adequate for a 6. Mine has worked fine for 15 K now behind my 235. The T-5 was used in many applications from 5.0 Mustangs, Jeeps to Vegas. They have the poorest reputation from the first two applications. The one you want for your truck should be from a 2 WD S-10 as the shifter in these variants is located nearest the front. It will locate the shifter about seven inches behind the stock 4 speed in AD trucks. Other variants will locate the shifter in the bench seat. You can use another variant and change the shifter location by bolting the tail shaft housing from an S-10 to it. The shifter is part of this housing so it will be located in the S-10 position. This seems to be your only option if you want a close ratio box from a V-8. The close ratio uses a 2.95 first gear and is best if you want performance set up. 2.95 is what the stock three speed uses for first in the '50. The info I got from the Chevy dealer shows 2 ratios available from '85 to '90 in the S-10 though there may be others. ML2 code in the glove box is 4.03 first and 2.37 second. ML3 is 3.76 first and 2.18 second. I use an S-10 box with a 4.03 first. 4.03 is great for occasional steep driveways or creeping in a field but I usually start off in 2nd gear, which is 2.37. This will require some slipping the clutch at takeoff but works good for me. However, it will not give you a great launch if you want to do stoplight competition. Another option is a box with 3.55 first gear. To me this seems to be in-between the two and a compromise but I have heard some say they like it with a 3.73 rear. I prefer a 3.90 rear which will cruise 65 at 2100 RPM but will also pull hills OK in OD at 50.. Also watch your OD ratios. I ended up with one early model box that had a .9 OD ratio. Hardly worth shifting into OD. Most are .72 but best to mark the input and output shafts and count revolutions. An early box with mechanical speedo is a plus. I hear it's very expensive to change from electronic to mechanical speedo drive. I find S-10 box's plentiful for $50 to $100 at swaps but the V-8 variants with 2.95 first seem to run more, around $200. Junkyards are very expensive places to get these. If time is on your side, shop around. Penny saver type papers are also a good source.
After, or preferably before you purchase, it's good to take a peek inside before you install. You can't just pop a cover off to inspect these, it requires you pull off the tail shaft housing and shifter to get the top cover off. As long as you're doing this you may as well at least re-shim the end play off the main shaft. They use tapered roller bearings and any end play will result in sloppy bearing clearance as in sideways play. One might consider bearings and synchro's at this time also. Instructions are in Chilton's at your local library. Also good time to change the speedo drive gear if necessary. You might not have to do this if you're using a 3.73 or taller rear but I did with the 3.90. The S-10 used only one drive gear (9 teeth I think) but I found the Camaro T-5 had the right gear. I used GM P/N 14071731 (7 teeth drive gear) and 14077086 (driven gear) for my ratio of 3.90 and tire size of 28.8 inches. This will vary for different applications. Also remove the C-clip from the rear of the output shaft. I don't know the purpose of it but found it interfered when I used a particular drive shaft; could not slide it far enough forward to install.
Some simple mods will need to be done to the tranny. Ream or drill the 4 mounting holes to open them up from the metric size to half inch. You will notice the input shaft, the pilot part of the shaft and the front bearing retainer are longer than the stock tranny's. I cut about 1/4 of an inch off the pilot to match the stock one. This still leaves the overall length of the shaft a little long but was not a problem. You can drive the pilot bushing in further if it does interfere. I also cut the bearing retainer to match one from a stock tranny. I found the tricky part was putting a clutch package together. Notice the splines are shorter than your stock tranny. They are also different size and number. S-10's are likely 1 inch 14 teeth, V-8 boxes are fine spline. Mine was 1 inch 14 so I used a 11 inch flywheel and an 85 Astro Van 11 inch disc with a pressure plate from a small block. Used a throw out bearing from an SB but for a cast iron fork. Use the stock fork and clutch linkage. This setup put me within about .100 of running out of clutch splines on the input shaft. Enough to dis-engage but barely. I think if I ran out of splines I would have ground down the clutch disc hub a little. This is a gray area and I recommend setting everything up without a pressure plate installed so you can see what's going on for clearance. I found some Astro Van discs very different from others. Any other make disc could work as long as it matches your input shaft and is the right thickness.
The gearbox is a couple inches longer than the stock one so you will need a shorter drive shaft. For my 50 with the 55 rear I found an Astro Van shaft fit perfectly with a Napa 348 cross over U joint for the rear. Again, this may vary for your application. Also needed a longer speedo cable. I've run the T-5's and Saginaws with no trans mount several thousand miles with no problems, the trucks mount on the bell housing. It did however make me nervous with aluminum case sticking so far out in space so I fabricated a cross member from the original truck's cross member. I cut and welded a recessed pocket in it, also cut and welded in tubes where the bolts pass through the vertical part of the cross. I bolted the mount to the tranny first and then slid the cross under it and bolted it on. Again perhaps not necessary but makes me feel better.
Most people at shows think I have a stock 4 speed. I used a 4 speed shifter welded onto the S-10 shifter. This does make for a long throw but I've heard of people reoperating the shifter pivot to address this. I acquired a Hurst after market shifter with a short throw I plan on installing.
With the T-5 I can cruise 65 at 2100 rpm and still pull hills at 50 without having to shift out of OD. . I don't hesitate to drive to a show or vacation hundreds of miles away. 65 to 70 is about all I feel comfortable doing in the truck and I still prefer a state highway to the interstate. Ratios are a gray area of this article. I make recommendations based on what I like and others may prefer different ratios depending on geographic location, engine, cam, how you plan on driving, towing, importance of gas mileage vs. performance. This is not a straight forward install, particularly the clutch and beginners may not want to tackle this swap.