Well I havn't done what you are suggesting, but I can make some guesses how it might work. The engine needs to run up near 200F to operate correctly, especially if you are burning other than #2 fuel oil. This means you will be adding a certain ammount of 195F water back into your system. Can your system deal with this added temp?
A 6/1 needs to dissipate 17,500 BTU/HR thru the cooling system. 4 GPM is 32# per minute or 1920# per hour. 160F raised to 195F is 35F change. That multiplied by 1920 LB/HR = 67,200 BTU per hour that could possiibly be removed from the engine. So this should be possible. At a 35F delta, the 6/1 would heat about 500# or 62.5 gallons or 1/4 of your total flow, of 160F water to 195F per hour at full load.
I would use a thermostat on the engine output to maintain 195F. You will also need a bypass with a slight restriction so the water wants to flow thru the engine when the thermostat is open. This might take a little trial and error to adjust so that the engine gets adequate flow to carry away it's heat, without overly restricting the bypass line and causing a system circulation problem, or overpressurizing the engine cooling system.
What is the pressure of your boiler system? I don't think Listeroids do very well with pressurized cooling systems. If it has much pressure, you may need a heat exchanger to separate the systems.
Ron