Ok this may seem a little odd and would need to modify the headers, I see a lot of aftermarket closed loop cooling systems fitted to marine engines. Some are just engine cooling and some are engine and manifolds cooling with the riser raw water cooled. I've been looking at upgrade manifolds and headers of late and just thought why could you not cool the headers with the closed loop cooling.
I do realize that some modifications would be needed to the headers to stop coolant from exiting the rear but can anyone see a real reason why this would not work as long as the heat exchanger is large enough.

Yeah it's a new engine going in so no problem there thanks.

I thought it lwould have been do thanks

As it's a new engine going in I won't be using the old stock manifolds, I guess I'm lucky being a Volvo Penta technician and rebuilding a lot of our Canadian government water crafts I have a load of 41 series heat exchanger To Hand which are more than big enough for the job.

I was hoping to hear from someone that has done it.

Only real reason is I have a chance to get a set of new fresh water Lighting headers as a trade for some work to a boat. I use the docks at work which is salt water also lake usage which is a little brackish. Thanks for the replies