I am with the others, water heat. Works way better then exhaust heat. I also used a flat plate with elbows. Mine leaked every time. Weld the plate to the manifold and end the problem once and for all or switch to a Clifford manifold with the water chamber already cast into the manifold.
Also, look at the bottom of your manifold. If there are sheet metal plugs staked into the casting (for the exhaust heat, you will need to weld plugs over them as they will also leak. Otherwise, the hot water will do wonders for drivability.
Forgot about the plugs!
I used a 2 part epoxy and then painted over them. They held for me but others couldn't get it to hold with epoxy.
