273 rebuilt but way low compression

Good candiate for a turbocharger or supercharger at low boost.
That is a common problem that people that buy project cars from someone that is just trying to turn a car and make a few bucks. All the hype they spew about fresh rebuild with new pistons usually covers up for not having real details about the car and parts used. You can put a 350 Chevy together with all new parts really dirt cheap if you try but it will be a turd also. Cheap parts can be compared to a Street Hook--. If you don't care what you end up with, don't complain. It's all about details.
Sorry. Sounds like you got told a truck drivers story. "You ain't gonna beleive this but........".