I have a 2005 29' Fountain with a 525 EFI. Water pressure is above 40 PSI over 4000 rpm. There's no blockage issue, and I'm thinking about installing pressure relief valve. Any one else install one? Any recommendations? Thanks!

I think the easiest way is to put fitting in top of sea strainer if you have the type that allows that , then run hose thru hull (dump that you will have to install ) need to use a one way valve so pump doesn't suck air at idle ( I use the ones from Kinsler , they are for there fuel injection system but have stainless guts ) just play with size off hose 3/4" should work for you .

If I'm not mistaking there is some kind of restrictor bushing in the headers to keep them full of water at idle.

could you elaborate on the restrictor bushing. Are you saying it is the reason for high water pressure. I run 40 at wot.

If you have those in there I would say its the cause.
Can you see the pics I attached above? If not i will post them in the
General D area. Also Merc makes a water relief valve that goes inline with
With the Water pump hose.

What drive?

I'm running a XR drive.

Stock lower?

Yes, stock lower

Pull the restricters out and test run to see if it drops pressure.