Thinking about you wanting to add manifold heat.
Ford trucks used a power steering cooler that might fit the purpose. It looks like a small aftermarket auto trans cooler with a simple U shaped tube surrounded by a flat fin package. Fabbing up an aluminum box to hold it snugly up against the floor of the plenum may do the trick.
Running it by itself without a full box around it wouldn't transmit as much heat to the plenum.
I never measured one but off the top of my head I would estimate the little cooler to be approx 6 to 8 inches long, 2 inches or so wide and less than 3/4 inch thick.
Just a strange idea that came to me before I had finished my first coffee of the day.
