It would have to be a new chevy duramax diesel in my 1968 chevy C30 motorhome.
I will confess that I’ve never done anything more than changing valve lifters and removing the heads of a 332 ci 1958 Ford sky liner. It was in such bad shape that we had to junk it. I did buy a 57 Ford that a guy had put a 1962 Ford 390 ci engine in. He used the original 3 speed standard transmission in it. It had so much torque that it would strip the teeth off gears of the transmissions that were designed when Ford had only the Y small block 292/312. I changed about 3 of those transmissions as it did not take much to break the gears. The guy I sold it to said that he thought it was a 406 engine. I will never know what size it was nor what the HP rating was, but it was too powerful for that 57 Ford. I guess the lesson is to consider the entire drive train when replacing the type of engine a car has.
The 2019 mustang EcoBoost Performance package would be nice in the A Model
BMW v-12 in my 1984 El Camino plus a T-Top would be one cool ride.
A Juiced( >500Hp) Porsche 993 twin turbo into an 87-89 911 or 930