Okay, here is the situation that I need some help with...please! twin GEN V 502 BBC with B&M 250 blowers hyd roller cams, twin Demon 750 blower carbs and MSD ignition with 30 degrees total advance. One engine runs perfectly while the other overheats constantly, I've replaced the entire cooling system but still it doesn't help even at lower RPM. When checking the plugs two of them where completely destroyed with electros missing from detonation or running lean but the jetting and all other parts are the same is the same on both engines. Anyone out there who can help with this mystery?

Well the obvious first, even though you said that you replaced the entire cooling system.
1. Sea water pump
2. circulating water pump
3. thermostat
4. Plugged up oil cooler due to failed sea water pump in the past ( good possibility )
5. exhaust manifolds - water port plugged
6. blockage in the water pickup port ( DRIVE - LOW OR SIDE PICKUP OR EXTERNAL )
7. Sea strainers?
8. Timing

Lots of things can cause overheating, but you need to fix it or you will be rebuilding that motor rather soon.
Give us some more information and we can give you some better advise.

from what I can tell it doesn't appear to be a water circulation problem and when I look at the water dumping from the exhaust on both engines it looks the same with the exception of steam coming from the hot engine. I've checked for blockage in the cooling system and it flow freely. I'm getting bad detonation in that engine but I don't know if it is from the heat or what. If I run at idle it stays cool but anything above 2500 rpm the heat goes way up.....I'm still stumped?

Take a look at where the water comes into the boat from the outside bell housing. The hose barb on the transom assembly - in the inside is held on with two 5/16 bolts - remove them and make sure the plastic insert hasn't collapsed due to salt build up around the outside of the hose.I fought our 45 for weeks thinking it was the heat exchanger gaskets, bad pumps, I back flushed it numerous times - and then while dry docked I pulled the drives to do a little maitenance and found I couldn't flow water freely from the inside of the boat out. I pulled the hose barb and badda bing there it was!! New hose - new insert - 160 degrees @ 5K. All is well..... Except now I've got to do the other one...... sucks. But now I know!!! Good Luck!

And Thanks to Ed @ClearLake Powerboats for taking care of it - on such short notice!!

I would say that the detonation is probably coming from the heat problem. It looks like txriverrat2001 has found a possible problem area. I've never seen that, but I'm not in salt water.
My brother had a similar problem and was caused when he hit a stump while idleing and it rammed a piece of wood in the water pickup. We thought we had it all out because we couldn't see anything. but somehow he ran something up the water passage and ended up get more wood out of it. Now it works perfect. Time to start looking past the engine!
Hope you get it figured out.

i got bit twice in this area(each engine), caused lack of water flow/volume at wot. I sure know better now $.

from what the engine builder i deal with says, when the electrode is missing but the plug is cocoa or NOT white in color, then its a heat situation. typicaly 1 of the 2 or both center cyclynders. this normaly indicates lack of water volume.
colud be one of the many issues stated above. or could be bravo fever.
you may want to inspect very carefully the water passages in the drive the gimbal and especially the water neck on the iner side of the transom assembly.