Are you measuring prop shaft center to prop shaft center or distance between the transom plates?
Big blocks work on 36" centers in side-by-side applications. Small blocks will work on slightly less, but you may be limiting your exhaust choices.
Staggered engines usually end up on 18" centers and a short stagger about 22" centers.
From your post I suspect you're talking about an old 28' Cigarette that had the Chevy 250 cu.in. 6 cylinder engines with Alpha drives. Yes?