I recently completed rebuilds on my 427 mercs, which included installing hei ignitions. I am looking for any gph input you folks might have achieved at 1000 rpm cruising speeds. My boat is a 1974 Concorde 35' flybridge. thanks in advance

It's safe to figure that you'll burn 1 gallon of fuel per hour for each 11 horsepower.
A flybridge boat is generally operating in "displacement" at 1,000 rpm.
So, my GUESS is that your 325hp 427's will burn 1.2 GPH at 1,000 rpm (13 hp each). Total of 2.4 GPH (guess).

1000 rpm is cruising speed?
I`m confused.Thats gotta be slower than a sailboat

This boat is located inland right now, but i may be making the trip to the coast...it came off of Lake Michigan, where it was used as a state authority boat previously. It has big props, and rolls pretty good at lower rpm...thanks for any pertinent info on a downstream trip of long duration.

I think you should be looking at it more as mpg. And that non efficient hull would be below 0.50mpg. (like my cruiser) and lets say you cruised at 10mph that would be 20gph or more.