You have a few options for plumbing the water-heated system.
1. Put T fittings in the heater hoses near the water pump (supply and return).
2. Supply: Use a GMC thermostat housing that has an outlet fitting on the generator side - I do not know if this is above or below the thermostat - I would guess that it is below. Return: to a T fitting in the lower heater hose, near the water pump.
3. Supply: use the adapter plate from Tom Langdon that goes between the thermostat housing and the head. This plate has fitting on the generator side. Return: to a T fitting in the lower heater hose, near the water pump.
4. If you have one of the original 261 water pumps that has additional out-flow and in-flow fittings on the distributor side, you could use those.
Some people have reported that they connected the return line to the hole for the drain outlet on the lower-rear of the manifold side of the block.
I have used the Langdon plate behind the thermostat housing and the 261 water pump that has extra fittings.
