If you are going to swap to a transmission that requires an open driveline anyhow, why not think about a 5 speed? The fifth gear overdrive'll make it nice on the freeway and the borg-warner t-5 that came in s-10's and blazers basically bolts right up to your bellhousing. I just modified a t-5 to put in my '51 and it took me all of an hour to measure, cut, grind and drill. Basically all that you need is a $15.00 4" angle grinder from harbor freight, which comes with a grinding stone and cut off wheel. You measure the length of the pilot bushing portion of the input shaft on the original tranny and grind down the t-5 to match. I took off less than 1/2 an inch. Then on the input shaft housing (the sleeve around the input shaft infront of the tranny) you do the same thing. I cut off around 1" of that and it took 15 minutes including measuring and marking it and checking the measurements and marks. Then what's left to do is to drill out the lower mount holes in the t-5 'cause the holes are a little to small for the original bolts. I'm using the clutch plate that came with the t-5 and the pressure plate/throw out bearing/flywheel that I already had. Once you've got those parts taken care of you'll have to remove the transmission cross member (it bolted into my truck, so was easy to remove). Then measure the driveshaft length and get the one you've got shortened and balanced, if necessary. Don't forget to set the transmission output shaft angle and axel pinion angle. The shifter comes up to the cab about 6: back from the original so you'll have to deal with modifying the transmission cover somehow, I'm using a 3-spd cover and cutting a new hole. (about $40 from ChevyDuty). After that, if you want, you could weld the orinigal shifter from your truck onto the shifter from the t-5.
All in all, with the tools and parts in hand, maybe a saturday job. A benefit beyond the overdrive was less weight, a back up light switch from the tranny, better gas milage, easy to find parts for the t-5 (rebuild kits are $125 for everything!), etc...
