injectors

This is what's called "Opening A Can Of Worms" Seriously when you take a injected engine and remove the necessary sensors and the computor, there is no way for the injectors,distributor etc to know what to do to control fuel and spark delivery. I think you should get someone knowledgeable with the time and PATIENCE to help you and/or seriously consider converting to a carb and a simple HEI system or a aftermarket computer and harness assembly if running a injector system.

I was thinking the same as ProChevy. I am sure Edelbrock have an intake manifold and downdraft carburetor that would make that boat fly. If you do go that route the fuel injection fuel pump pressure would be much too high for the carburetor. Have you checked fuel pressure and fuel delivery, (fuel injection should have about a pint a minute), could that be why power is falling off?