I learned this trick from the jet boat guys 25 years ago. If you route your raw water through your exhaust manifolds first, then into your engine, you pre-warm it. Then all you do is put a drilled thermostat in and you have a bulletproof cooling system that never runs cold. On this setup, the engine outlet water runs to your exhaust tails.
I like that idea. Could you route it like this (1) bottom of one manifold traveling to the top (2) jump across the motor to the top of the other manifold traveling down (3) enter the engine via crossover (4) finally exiting thermo housing with a feed to each tailpipe. What do you think?