Many years ago I bought a boat with a 4-108 in it and experienced the same overheat issues which you are discussing. At the time I was employed by a major marine engine
manufacturer (since retired) and I worked with our heat exchanger
supplier to build me a one off heat exchanger to solve the problem.
The standard Perkins
12 inch heat exchanger that was mounted behind the engine
had only a 9 inch tube bundle inside it, and the one we built used a 12 inch bundle, for an increase in heat rejection capability of 33%. This solved
my problem. The new unit bolts in exactly where the old unit sat, on the same bracket, and connects up almost exactly the same. The overall length is 16.5 inches long and all of this extra length extends 4.5 inches toward the starboard side of the engine. This unit worked very well and controlled engine temp regardless of engine load.
I still have this exchanger and would be happy to sell it for a reasonable figure, but more importantly if there is sufficient interest it may be possible to arrange to purchase
a number of units from the manufacturer at a wholesale price
If you are interested, PM me and include your e mail address. I'll send you photos of the old and new HE, the new and old mounted on the engine, etc. We'll see what we can make happen.