Yes the manifold will bolt on. If you go with a GM Performance block which is used by Mercruiser and it's a one piece main seal hydraulic roller block . the fly wheel will bolt on. The rotating assembly you have is an internal balance on the front and external balance on the rear. You can get a 377 internal on both sides.In that case you'd have to get a zero balance flywheel. So That's something you have to ask, from whom ever you get the long block from. What I suggest is going with the short block with a compression ratio based on 64 CC aluminum heads and a HR 270 high duration camshaft. Then buy a good set of USA made aluminum heads and a marine roller cam