People say marine engines have heavy duty valvetrains and a different cam shaft to help the engine survive the high rpm cruising speeds. This may be true for some engines, but I do not believe it true for the SBC. The only difference in the structure of the engine is that all marine 350's are 4 bolt main blocks to help with reliability at those high rpm cruising speeds. You will need to change to brass freeze plugs as well as stainless steel head gaskets if you are going to use raw water cooling. The bolt ons like the intake manifold, carb, spark arrester, exhaust, distributor, etc... will all carry over from the 305 to the 350. I do not recommend you reuse the 305 heads as they have smaller combustin chambers and your compression ratio will be very high. You will need to get a new prop with more pitch to efficiently use the added torque of the bigger engine.