I have a 1965 C-10 with a GM Performance Parts 350HO and a Turbo 350 tranny.  Previously the truck had two 700R4 trannys burn up in it.  The Turbo 350 was a fresh build by AAMCO and after 6000 miles the sprag is broke.  Tired of shops giving me the go-aroung, I am going to tackle this problem by myself and have some questions:

Besides sprag replacement, what other goodies should I include in the rebuild?

What would be considered a "clean" environment to work the tranny?

Should I even be doing this? Should I just save up for a 4 or 5 speed manual?

Any suggestions are appreciated!  

You need to figure out way they have been failing. Sprag failure is usually for ruff treatment and hard shift kits. Also tall gears and a strong engine can place a lot of shock load on tranny in hard shifts. 700R4 are not bad trannies and when they fail is is usual because of one of two thing (or both). The first being heat for it not being cooled properly and the second is from too tall of axle ratio because a 700R4 has a 30% OD and unless you have it geared proper in axle what happens is that tranny is forced on to stall a lot when cruising under load and this generates a lot of heat and slow bakes tranny. I would suggest that you go back to a 700R4 (make sure it is a 30 input spline one not a early 27) and regear your rear axle and install a aux cooler too. I have a 89 4x4 burb that I bought new that has about 185K on it now and tranny still works like new. It has towed, been across country and rockies more than a dozen times and seen temp from colder than 40 below to above 105. I have changed fluid and filter in it every few years and it does have a aux tranny cooler and it is not geared tall for tire size either. With stock tire sizes a 3.73 is minimum you want with a 700R4 in a P/U with a strong motor and 4.10 are better still. But even with 4.10's your cruise RPM will be lower in OD than with a THM 350 and 3.08 gears.