What is in the comet? A 302 will go, but you'll need the trans for it too (bellhousings changed in 68 when the 302 came out)

Not sure of Comets, but come fords run the shift linkage (on autos anyway) between bellhousing and frame, and chanigng trans will mess this up beyond a simple fix.

Here i can get a 70 302, with truck attached, for $250. Well, ok, red paint over rust. there isn't much left that would actually work as a truck!

As to your car...if you could get a totaled or thereabouts running car with a 305 or 350 (or a truck, but less likely) then you could swap it over. It IS possible...the Camaro mess, (did i type that?) was a $750 purchase with a reman 305 with 10k on it, and a new quadrajet, TH700R4. The 80's camaros/'birds are pretty cheap right now, and getting rusty.