I bet it has small block motor mounts. I'm not to proud to say that I used a block of wood and a sledge hammer on my 1968 Yeller Camaro when I was 18 to get an hei to fit my 355.
My big block now is installed with 302/350 mounts to center the driveline and my msd distributor is snug but servicable. No way an hei would fit.