My truck originally came with a SM420 and I replaced that with a T-5 from a S-10. This was a fairly easy swap, but it wasn't a bolt-in. Here are the issues I needed to deal with:
1. I had to replace the clutch disc with a 14 spline disc from an Astrovan.
2. Transmission mounting holes had to be enlarged slightly to clear the stock 1/2" bolts. The lower two mounting holes on the T-5 had no threads, so I had to use longer bolts with nuts in those holes.
3. Input shaft and bearing retainer were too long. A commercial spacer plate is offered to solve this problem.
http://hamiltonintakes.com/products/...5-swap-adapter
My time is a lot cheaper than my money, so I shortened the T-5 shaft and retainer myself. Other people who have done the T-5 swap reported that they did not need to shorten anything, so you may not run into this problem. If your T-5 shaft is too long you will know immediately when you compare it with the one on your 3 speed.
4. I had to get a different driveshaft. Any driveshaft shop can make up the shaft you need, but I was able to find one in a salvage yard that worked fine after I changed the yoke. Here again, my time was cheaper than my money.
I swapped the T-5 only because my truck already had a floor shift. If it had been a 3 speed with a column shift, I would have used a factory SM318 3 speed with overdrive instead. This is very similar to the transmission you have in your truck now, except that the overdrive unit replaces the tail housing. The SM318 overdrive transmissions were offered on Chevy passenger cars and trucks from 55 to 65 and are not difficult to find.
The overdrive 3 speed is an easier swap than the T-5. You won't need to do anything with the clutch disc or input shaft. Depending on the transmission year, you may need to swap the driveshaft or driveshaft yoke, and that is all. If you get a car transmission you will need to use bolts with nuts in the lower mounting holes as with the T-5.
If you need more info, please feel free to ask.
Ray