351 windsor heat exchanger sizing

[Zappi]

My buddy has a 28' Trawler type cruiser with a 351 Windsor v drive powering it. He recently bought the boat and it has a fairly new heat exchanger on it. She overheats on anything more than an idle. Every piece of the coolant system has been replaced or cleaned but no change in temp.
My thought is the heat exchanger must be too small.
Is there a predetermined size that should be used for this motor?????
Thank you!

[gonzo]

The coolant water comes out of the exhaust. Are the manifolds and risers clean or corroded and clogged? They can prevent the raw water from circulating and overheat the engine. I assume the water intake is clean and the raw water pump was overhauled too