273 Mystery Motor

the block doesn't have anything to do with the push rods. the type of cam and valve train gear are what determine your pushrod selection-- in this application.

pull a valve cover and verify what you have before making any choices.

there are several options to weigh here: is it a commando motor? do you care if it is? do you want or care if it's hydro or solid cammed whether it's a commando or not?