By the time I got finished writing this up (including a couple interruptions from wifey) you guys had carried on a good deal of dialogue, but I am leaving it as it stands. Thanks
I would think it would be more expedient to plumb a sort of "by-pass" arrangement into the hose rising up to the radiator so that naturally hot water could rise up through the regulator/vaporizer.
This is going to require that the hose in place now be cut at 2 places so a short section of pipe can be inserted into each radiator hose.
One pipe in the ascending hose near the engine head and the second pipe in the descending radiator hose somewhere near where the hose attaches to the block.
The coolant ports on the J-model regulator are 3/8" NPT and as such your 2 sections of pipe to "tap into" the radiator hoses would need to have either threaded fittings welded into them, or have 3/8" heater hose nipples welded into them, to transport the hot coolant to and from the regulator/vaporizer.
The regulator needs to be mounted above the point at which you are feeding coolant into it.
Because the net effect of the regulator absorbing heat from the coolant and transferring the heat to the liquid propane. the coolant temperature will be lowered in this process and will induce its own convection cycle circulation loop. Hot water in from top hose and cooled water falls back to the lower region of the engine block.
Ideally you would also add a VFF 30 vacuum operated fuel lock off valve between the LP tank and the J-model regulator to insure that no propane flows in the system if you forget to turn off the tank hand valve.
Legitimate LP engine fuel systems are required to have an automatic fuel lock off feature that senses engine vacuum or oil pressure so that the system automatically shuts of fuel if the engine quits running with the key still on. You can't just have an electric fuel lock off valve that is energized when the key is on, there must be an interlocking switch that senses engine vacuum or oil pressure before activating the lock off.
A direct vacuum operated fuel lock off such as VFF 30 does the job nicely without any electricity involved.