I have used water heat and exhaust heat with Fenton dual exhaust manifolds and an Offenhauser dual intake..
They both work fine.
The water heat works best if there is flow of heated coolant.
Some two-tube plates do not seal between the input and output chambers on the base of the dual-intake manifold.
The Offenhauser dual intake is the only repo one that I know of that allows water/coolant to flow from/into one end of the intake and then through the manifold and out the other side of the intake manifold (in/out at the base, using a correct-designed two-hole plate/seal). (Clifford might sell one, also).
I decided not to use coolant water on one of my 261s with dual carbs - I did not want to worry about a coolant hose/fitting leaks.
Many modern "repo" bottom plates are for one tube (one heat line to a plate on the base of the intake manifold - from from one of the Fenton exhaust manifolds) - there is no flow, just heat exchange. Works just as well with exhaust heat or coolant.
Exhaust heat also works fine with a 1960 261 on my 1954 Suburban. It has two exhaust tubes from the Fenton dual exhaust manifolds to a plate on the base of an original single carburetor intake manifold (two heat lines, for a balanced appearance).
