The old Crusader manifolds are a consideration for the SBC motor. They're taller, have the water dumps all the way back, angled out and will also give you a performance gain. Should be the same width as the merc manifold- maybe slightly narrower. if you have the room, you can use spacers from 3/4" plate to allow you valve cover access. If not, you'll need 2-piece units. Someone used to make jacketed stainless tails for them that blocked the water exits at the rear flange. These were a class-legal upgrade for P class way back when.
I wouldn't do anything until I knew for certain what was in the motors.