If your carb is on the small side for your motor, use an open spacer on the dual plane. Shhhhh, speed secret.

If you have plenty of carb, then a spacer won't do much on a good dual plane.

What is this going to have for a carburetor ?

850 demon or 9022 800 holley

Own the demon already but it's been sitting a little over a year.

Yep, it lived just fine. Like SB said, I had the 3 inch riser blocks on my stock Merc exhaust, which likely helped prevent water sucking. I ran the ZZ502 hard for 4-5 seasons and it ran like a Swiss watch... flawless. I know zip about the 502HO but if it's built to the same standards as the ZZ502, and if it hasn't been opened up and fk'd with, I'm pretty sure all you'd need to do to get on the water is change the bypass valve in the remote oil filter pad, step up to a bigger oil cooler, get a decent exhaust system, bolt on a marine carb that's the correct cfm for the CI, and go boating.

I need a good carb. Thinking the Holley 9022 800cfm would be perfect

I did a similar upgrade to my 260 Baja a few years ago. Went from the stock Gen 5 454 to a Gen 5 502 Mag. I used a Performer RPM manifold, a Holley 9022, a DUI distributor with 24* advance and 12" x3" oil cooler with a separate 5" x 2" PS cooler. Used the stock cast iron exhaust. The jetting I ended up with after much trial and error was 87p, 98s. The stock jetting in the 9022 was 72p, 87s. WAY too lean. You should be good with the dual plane that comes with engine. I would swap the water pump for amarine unit with a stainless steel impeller. My only concern would be the cam. Would want enough LSA to not get reversion with stock exhaust. I could not be happier with my swap. I have over 200 hours on it now. Really woke up the boat with a 24 pitch Bravo 1.

I also changed the oil bypass valve to the 30 pounder from a 6.2L diesel. And I used the 1" carb spacer that was on the 454. Good luck.

I ran mine w/ the stock exhaust and ignition, one inch spacer w/Holley 9022 Carb, did not change anything else, it topped out at 5100 rpm, it ran perfect and dependable, I did not want a hot rod engine, I wanted dependability on a low budget ,

What he stated about the spacer on a dual plane is correct. I support that with dyno experience. SBX if your set on using a dual plane, best one to use (as previously posted by others) is the Edlebrock Air Gap 4150 based manifold.

Take a good look at the HP950 Holley. It has a 750 throttle plate. This is really a 850 hi velocity---sorta. Again, I had dyno time on the carb too. Don't be confused with the CFM numbers or model numbers issued by Holley. Many times they do not make sense as to the true carb in the box.

502 HO Technical Information

Horsepower ..................... 450 @ 5250 RPM
Torque .......................... 550 Ft. Lbs. @ 3500 RPM
Max. Recommended RPM ..... 5500
Compression Ratio ............. 8.75:1
Block ............................ 10237292 Cast iron - Four bolt maincaps
Crankshaft ...................... 10183723 Forged steel
Connecting Rods ............... 10198922 Forged steel -Shot peened, 7/16" bolts
Pistons .......................... 12533507 Forged Aluminum
Piston Rings .................... 12524293
Camshaft ........................ 24502611 Hydraulic rollerLift: .510" I, .540" E.Duration @ .050" 211o I, 230o E.
Timing Chain ................... 3891519 Single roller
Cylinder Heads ................. 14096801 Cast Iron118cc Combustion chambers
Intake Valves ................... 14097045 2.19"
Exhaust Valves .................. 14097049 1.88"
Valve Springs ................... 14097002
Rocker Arms .................... 12523976 1.7:1
Intake Manifold ................. 3933163 Aluminum with Holley flange
Torsional Damper .............. 10216339 8"
Flexplate ........................ 10185034 14"
Valve Lash ...................... Net lash
Spark Plugs ..................... MR43T Gap .045#10 Rapid Fire Gap .045
Fuel ............................. 92 Octane
Ignition Timing .................. 36° @ 3600 rpm With vacuum advancedisconnected

NOOO Use an HP Holley carb. Before you keyboard captains sound off, "now hear this". Send the carb off to a carb shop. For lunch money they will install the correct marine vent tubes. Bam, you now have an HP marine carb.