Did the truck have a 350 and the standard tranny in it to begin with?
The 4-speeds with granny are solid trannys and they wil last dang near forever. Same can be said about the 350 motor. Good solid and dependable. I've known several mid80's 350's with over 200,000miles on them.
So the reason why I ask is do you know exactally how many miles are on the truck for it to have 60,000 on a new engine and 30,000 on a new tranny?
If I had to guess I would say ~250,000miles on frame, axles, etc. I would check everything over real good, diff fluid, etc. If 4wd, check out transfer case, front axle, u-joints, etc.
Don't sweat the jasper engine. They are as good as a brand new gm motor. I'd question the tranny. Is it bran new? or Rebuilt? If so, who rebuilt it?