You're right, the flow/ temp rise is critical here. It seems to me you'd
have to just figure it out by trial and error, ie., wrap more soft copper
tubing around the manifold if you want higher temp rise, vice versa. The
problem would be too much heat if you're using the exhaust manifolds (you'd
have to install some sort of a shutoff device to keep from producing steam
all the time) and you'd have to use compression fittings rather than
sweating the copper as solder would melt. All in all I think it's safer to
use the heater pipes. There are any number of heat sources you could use
from the engine to accomplish the same thing. The exhaust manifolds would
only have the advantage of near-instant heat in freezing weather.