[Mopar Guy in Delaware]

I believe I'd go to a 360 from a wreck that was already operating as a full unit. Engine trans and all. Truck units are usually beefier and all that but a little weight gain takes a tremendous amount of horsepower to overcome. Leave the truck motor in the truck.
You need a good smooth, lean powerplant. Actually the 273 could be made to perform well. What's it equiped with now and what are the problems with it. There's a lot to be said for a good fitting engine and trans installation. If its stock and unmodified, you may have the best unit there now for that car. A 273 with open intake, 650 holy, headers, four speed or tight torqueflite with mild cam and mild rear gears will do you well for a lot less than replacing everything, trying to line it all up, and then starting to tune the engine. Just my ideal thoughts. Good luck. Plus, mild cams are not expensive.