Re: non vortec to vortec engine swap
If you run a carburetor setup, fuel pump should not be a problem. The reason I think that entered the discussion is because carb'd engines only run about 7psi line pressure where Vortec fuel injection needs about 60psi for the injectors to operate properly.
I doubt a Blazer cam will work - seriously doubt it. You would need to have a spec sheet on the cam, or degee it to verify NO cam lobe overlap to be sure. Water ingestion through the exhaust would kill your new engine right quick, and it will kill it ala broken piston/s or bent rod/s. I don't have a good feel for whether you just want to swap heads OR if you want to drop in the whole Blazer engine and make it work in a boat, but suspect the later. If so, you'll need a MARINE master gasket kit that will have marine gaskets and brass freeze plugs. Vortecs run roller cams, you probably have a standard cam in your current setup.
Vortec heads have intake bolt holes that are vertical whereas the earlier heads had angled bolt holes. I'm not sure offhand on the TBI motors that were in between production carb'd engines and Vortecs. To 'do it right' you may need a new intake manifold with vertical holes. I have heard of guys using a drill or ream to modify angled bolt intakes to bolt onto Vortec heads.
If you run the Vortec block, that means you will have a 1-piece RMS and I think the crank bolt pattern for the coupler is different between 1-piece RMS crankss and 2-piece RMS cranks. So you may need a new coupler, which probably isn't a bad idea to replace anyhow as you will have the engine out.
You may have to run higher octane fuel with the Vortec heads due to higher compression but the lower operating temp of the engine in a marine environment may help prevent pre-ignition.
You may want to consider replacing the timing chain & gear while you have the engine out and oil pan off. Replacing the oil pump may not be a bad move either. The aluminum Vortec oil pans are "structural", which means they bolt to the transmission bell housing in an automotive environment; oil pan has flat flanges on the back of it. I don't think this will cause you problems with the Mercruiser bell housing, but you never know. If it does, there should be enough meat on the oil pan to modify it with cutting discs / grinder, but don't be surprised if you can't get the Mercruiser bell housing inspection cover to bolt up slam-dunk style.
