Need some good advice on engine replacement

[woodrow59]

My '68 Longhorn has a TH400 which I'm guessing is original (don't know for sure). Were the Longhorn transmissions different or just the drive shaft ????

Previous owner put in a '85 305 which is gettin' ready to bite the dust. Been lookin' around for old cars for a donor engine (cheap) .
What will bolt up to that TH400??? Will all Chevy SB's bolt up? How 'bout Olds or Pontiac ??

I have limited funds so I need to find a cheap way to replace the engine. My mechanical skills are above average (I have rebuilt a carb, replaced everything on the outside of a motor and fix anything around the house that my better half says needs fixing LOLOL)

But,,,,,I have never replaced a motor. I know I can do it,,,,,,,,I just need to know what will bolt up to that TH400 without any modificactions.