If you have a smallblock, locating a used A500/518 should cost a few hundred and some fair amount of fab work in an A-body.
You can decide to have it rebuild first or take the chance to install it right away.
I've done it twice without rebuild with success.
Currently fully rebuilding an A500 adn installing a TransGo Repr. kit, but I could have just installed it after a band-adjustment.

Things get more expensive with a bigblock.