I want to add that placing the J-model vaporizer near the lower end of the ascending radiator hose will cause a convection loop to form passing through the regulator as first choice (instead of climbing higher to fall down through the radiator) if the coolant return hose from the regulator begins to descend immediately toward the engine block.
Because the new J-model circuit in the plumbing provides a sort of "bypass the radiator" condition..........you will want to keep an eye on the coolant/engine temperature to make sure that adding the vaporizer heat loop does not decrease flow volume through the radiator enough to cause engine overheat.
Ways of compensating and "fine tuning" the vaporizer heat loop could include adding a restriction orifice fitting into the vaporizer coolant plumbing.
But the easiest way to handle this would be to use an extra long length of 3/8" heater hose for the returning coolant line (going to the bottom of the engine block) and simply raise that hose to a higher elevation if it seems that the radiator is not receiving sufficient flow volume to affect adequate cooling.
Liquids will seek out the path of least resistance if given unequal elevation routes to flow through, so the return hose from the regulator would be the path the convection loop would want to use if it is lower than the loop path through the radiator.
You might want to set up your return hose out of the vaporizer so it rises to the same, or maybe 1 inch lower than the normal level of coolant in the radiator top tank.
I don't know from experience how much the flow rate of a convection driven cooling system is, but I'll bet it can't be all that much. Or in other words, adding a 3/8" alternative loop might "reroute" enough flow volume of coolant to decrease cooling in the radiator (despite the fact that the vaporizing propane will provide some cooling itself) so you really want to route that return hose strategically in regard to WHERE it changes from ascending.......to descending.......in the scheme of plumbing.