RBT,
Check your vacuum at 700-750 in gear. Adjust the PV as necessary....possible you may need to go to no PV and increased jet sizes.

May also need to adjust secondary butterfly if he's not running a PV. I've seen a lot of carbs without PV's load up when using big cams without PV's because the primaries have to be open more to idle at the correct rpms and the carb actually comes off the idle circuit and starts pulling fuel from the boosters.

Recently ran into a similiar problem with this boat.... It's a 454, 9.6 comp and an 850 holley and 741 cam, which in a 454 makes it hotter then the same cam in a ZZ 502. It didn't have a PV when I started. When I took the carb apart I found that one of the primary jets came loose and fell into the bowl. I then jetted down the primary and installed a 6.5 powervalve. We also went from a #8 NGK race plug to a #7. The engine idles all day long now, no fouled plugs and you can beat the snot out of it.

http://media.putfile.com/Billys-JC-Performance-Motor

Its a 22. I have a 496 and I don't remember the duration(236-240?) but the lift it was around .610 lift on both intake and exhaust. I have the same heads also. About the only difference between engines is I have a Barry Grant carb.
I never put it on the Dyno.
Brett did my prop last spring. Worked great, 25 Mirage plus. I turn it around 5,000 at 77 GPS

The tail pipes in the picture, are those for swim platforms?

Yes, the pipes are for swim platforms, with the bottom turned out to exit the sides rather than back.
I have a stock 26 bravo and turn it 5400 with the 1.50 gears in my Bravo.
What drive do you have?

Aluminum heads run colder - change the t-stat to a NEW 150 and you're probably on the right track.

I am no expert, but I think a vacum of 4.5 is too low. Even with the cam. Where are you getting the reading from? Check out:
http://sbftech.com/index.php?topic=3020.msg29472

Did you have any backfires? The rich condition could be a blown powervalve.

Personally I don't think a crossover would cause the front two cylinders to run soooo much cooler than the rest. I ran a crossover but went back to circ pump because I could not get the temp over 130. But I loved the look and simplicty of no moving parts, I just wanted my water at 160 and more importantly, my oil above 212.

I am also running a crossover. Oil never gets above 180 and water temp never above 130. Any long term problems?

there something going on with only 4.5 vacume, you need to look there, as stated before, if you have a power valve(and you should) and it's anything over 4.0 it's open at idle and will cause a over rich condition....but that vacume seems way to low for that cam and set-up...Rob

i agree, do we have a vacuum leak below the carb somewhere? i would think he should idle with 10-12 in vac below the carb not just 4.

.02