273 w/ offenhauser 4 barrel intake & 904 PB

A bit confused. You state it was rebuilt only 1000 miles ago, yet you don't know what pistons are in it and were told the intake has been on it for 40 years.

Was it rebuilt 40 years ago and not driven more than 1000 miles since?

By "rebuilt" do you mean a few changes without removing the heads like the cam you mentioned? A cam swap has never defined a "rebuild".