I have a 454 bored out with hp500 roller cam and rockers, wanting to upgrade the q-jet carb (having problems)and cast stock intake and upgrade peanut heads and exhaust later found good deal on a holley 850 and a single plane intake could I use it or am I asking for trouble? Boat is a 272 Baja single engine....

Those heads won't work good with a single plane intake and 850 carb. Not so sure about that cam either. 750 with a dual plane would be best for those heads.

I would Use the carb and get a dual plane intake.