What pressure does the radiant cooling system run at? The lister/listeroid cooling system was designed to be open/unpressurized. I believe some have used automotive radiators up to a pressure of about 15 PSI, but most don't reccomend it and certainly wouldn't reccomend any higher pressure.
I too would reccomend a heat exchanger so you could use automotive coolant with it's corrosion inhibitors to prevent corrosion and mineral buildups in the cylinder block and head. This would also keep corrosion generated by your cast iron engine from possibly contaminating your hydronic system.
A heatexchanger can be run thermosiphon, I am doing it just like Lens described. I just uploaded 2 new pics of my current installation to my coppermine gallery at
http://listerengine.com/coppermine/index.php?cat=10113 There is an older pic there also when I was first experimenting with it this summer, that shows the back side. It is small! Mine is a 5" X 12" 10 plate brazed stainless flat plate heat exchanger with 1" male NPT ports. I paid about $98 for it on E-bay from a outdoor wood boiler supplier. The stainless plates are bazed together with copper and they are rated for several hundred PSI. The herringbone plate pattern gives a very high surface area for it's size and also has high turbulence with low flow restriction at low flow rates so it is very efficient with slow thermosiphon flow.
Unfortunatly your problem with using a heatex is the available delta or difference in temperature. I designed my system for a 75F difference to give me 120F water to my domestic hot water tank with a 195F engine coolant temp. At a 3KW electrical load, mine does this at 3/4 GPM with 75F water feeding the heatex secondary loop. 75F to 120F is 45 degrees. 3/4GPM is 6 pounds of water for a heat output of 270 BTU/MIN or 16,200 BTU/HR. Your 12/2 should sustain double the electrical load and output twice the heat at that full load. But your best case with 174F hydronic temp is only a 21F temp difference from the 195F engine coolant temp. The lower the heat difference, the lower the transfer rate. You would need to make up for this with a larger heatex surface area. With a heat output from the engine that is twice mine and a temp difference of around 1/4 mine, I would estimate that you would need a heatex approx 8 times the size of mine(80 plates?)...
Another issue, particularly with thermosiphon is overall heat difference. This heat difference and the changes in water density that it creates is what powers the thermosiphon flow. The water comming out of the primary loop of the heatexchanger will always be higher than the secondary loop input. What is the water temp comming back from your hydronic radiators with the 170-180F input? Unless it is down in the 80F range or less, I don't think you will be able to maintain enough cooling across the primary loop to maintain thermosiphon. You really need the water returning to the engine to be under 100F. My thermosiphon flow starts to slow and falter above this point. I wound up raising my heatex approx 12" higher than my original plan for this reason. I just didn't have enough cool water on the down side comming out of the heatex to power the thermosiphon process properly and it would falter at high loads. If the water comming back from the hydronic loop to the engine heatex is too warm, you may need a pump on the primary loop to maintain adequate flow thru the system.
You also might run into transfer issues trying to mix these different heat sources. The engine coolant is a viable heat source, but it is also a device that must transfer a certain ammount of heat to function. Any hickup in this sceme can damage/cook the engine... Your description also leads me to ask what your alternate plan is for heat dissipation from the engine? I don't always need house heat, but I do always need hot water, so mine feeding the hot water tank works all year round. Now the excess heat once the domestic hot water tank starts to output greater than say 80F water from the bottom once it is at 120F/full, I can either run thru a small fan/coil radiator into the house, or one outside depending on if I need the house heat or not. The temps involved with the domestic hot water loop lend themselvs well to harvesting listeroid heat to maintain them via heatexchanger.
Good Luck with your project, it sounds like fun.
Ron