If you have a 4bbl carb you also have a 4bbl manifold so you should really stick with the 4bbl carb. Any carb you buy unless it is brand new might not be any better than the one you have. Usually they have to be cleaned out, new needle valve fitted, float adjusted, then fuel mixture and choke adjusted. Sometimes additional parts are needed like an accelerator pump diaphragm, depending on the carb. Looks like you have a Quadrajet, that would put it at about a 1988/89 at newest. No new Quads after that, so might as well rebuild what you have. With the Quad it is a little tricky to get the air horn off but after that it isn't bad for a rebuild. There is the choke linkage and the accell pump linkage to disconnect. There are many videos to show you how to do it properly. Those marine Quadrajets had the well type or divorced choke with the thermostatic spring mounted on top of the exhaust cross over on the intake manifold. The open a bit slowly, but work ok if the spring is ok and the linkage & choke plate are clean.
First though are you sure you don't have a clogged or rusted anti-siphon valve?