I'm wanting to replace the old Holley carbs on my 500 hp 454's.
I'm a big fan on Quadrajets but I've heard that at higher power settings they don't distribute the fuel/air mixture evenly across all the cyls due the the small primaries and large secondaries.

I've also heard the the float bowls can run dry when ran at WOT for extended periods.

Any thoughts on these issues?
What size Q-jet would work best on a heavy breathing 454? I was considering 850's. The current carbs are 760cfm.

Where are you planning on buying a quadrajet marine carb ,of any size

Didn't mercruiser use quadrajets?

They DID. It's not possible today to buy a new one

Not sure if Merc ever ran the Q-jets on anything with more than around 400 HP. I think there were some 800 cfm carbs on some early pontiacs, but they are getting hard to find, and might not work on a marine application anyway. Then you have to figure out what jets and rods to run with them, along with a plethora of other adjustments. All for probably little gain. If your Holley's are running fine, I would leave them. I am a fan of Q-jets on stock or near stock applications, as they are trouble free once dialed in properly, but they have their limitations. I do know that there are plenty of healthy drag cars running them in class racing, and they have a lot of tweaks that will make them perform, but by the time you spend that money, you would probably be a long way towards going with one of the aftermarket FI setups.

Mercruiser used them on everything back in the 80's 235 hp , 260 hp, 330hp trs, 365 hp trs, 4 cylinders , etc... Good carb. economical at part throttle. Now Hard to find. And yes Holleys are a better for race applications, but are not as economical.

After a bit more looking around, the biggest marine versions I can find are 750's. There's a number of them available on the web, of course they are used/rebuilt.
I found upgraded needle/seat assemblies that allow faster bowl fill, but still not sure they wouldn't run dry during extended WOT runs.

The Holleys I have now are the old 4011's. The idle quality is terrible. They're either idling too high or they're stalling the eng. Once on plane they are OK, but around the docks, they are a real pain.
I put rebuild kits in them, replaced the power valves and adj'ed the idle mixture numerous times, but no help. They gotta go.

I think some of the drag and circle track guys would modify or eliminate the anti-slosh baffle in the float bowl to give a little more capacity. I was running a regular marine Q-jet on a mildly modified 454 (making maybe 375 - 390 HP) and just changed the jets and rods to richen it up some, and never ran out of fuel. I think as long as your fuel pump and lines can keep up, you will probably be OK for an occasional full throttle burst. Now if you are running poker runs or your only speeds are idle and WFO, then you probably need to stick with a Holley anyway. What kind of intake are you running - does it even have the correct flange to accept a Q-jet spreadbore arrangement?

I used to have a spec sheet that had the rods and jets that were used on a 502 application. If I can find it I will pass it along.

If you stay Holley your best bet might be to try to find someone selling a pair of the HP500 Holley 800's. That would probably work well for your application. Merc used to build a blue racing version of the 454 that used the same heads, cam, intake, cam and exhaust as the HP500.

The Super Stock guys are running qjets up to about 900hp I highly doubt you will run out of fuel at 500 if you do it has to do with fuel delivery and not the carb. They made 750's and 800's the 800's are hard to find.

For simplicity I'd stick with the Holley base or maybe a BG.The problem with a lot of the alleged reman marine quadrajets is they aren't really marine carbs .They are modified automotive .A lot of reman "junk" out there in internet land.