I'm looking for a closed loop cooling system for my 500 EFI. I've located an aftermarket system but I think the system from a 525 EFI will fit since the only difference are the aluminum heads, so I understand. I'd really like to keep my engine all Mercruiser.

So, if anyone has a system, or a lead on one, or even the correct part number for a complete system please reply.

The Mercury 525 closed cooling has a heat exchanger that bolts to and flow's coolant through the heads, so it will not bolt to your cast heads, but maybe you know this.