I have a 454 .030 built to 420 specs but I am running a 750 eldebrock marine carb will I gain anything at all on top end if i switch to a 850 sea demon?

The 420s came with an 800 holley. So you might gain a bit.
But then again, the the 410hp 502s came with a 750 weber/carter/edelbrock, so maybe not. Does that answer your question?
Gary

I have a pair of 330 hp engines that were redone and dynoed at 469 hp using out of the box eldebrock 750 marine carbs. Dyno sheet showed I did not need more carb.

Thanks I figure I would just leave it alone

A wise decision!