I have a 98 312 fastech w/ stock 502 mpi's, At wot one engine is running at 30-33lbs of water pressure and the other is 28-30. I think I need to install relief valves but do not know where to plumb them in.

thanks

Thats what my 454 runs. All stock cooling system.

everyone Ive ever talked says it needs to be down around 15-20.

That would be ideal, but my 2000 502mpi's also run into 30-35psi range. They had 250hrs when I bought the boat and now have close to 600hrs on them.
I thought about adding relief valves, but figured its been fine for a lot of hours the way it is.

Don't think it makes any difference as long as you have some pressure.....10-40 PSI all work.

up to 35 is not too much of a concern, however if the boat were any faster it might be... rather than a pressure relief valve my suggestion would be to trim your water pickups a hair... start with about .030 off the deepest part of the tip... I'm sure this will help reduce your pressure to an acceptable PSI

Pull the exhaust risers and hoses, T-stat hoses, and descale the rust. Let it flow.