Yeah, the coolant plug on the side of the block is 1/4" (-4 AN). Though there are 1/4" NPT fittings with 3/8th barbs. The radiator is the ultimate return location for coolant. Pretty much anywhere you get coolant from the engine will return to the radiator.
The line under the thermostat should be the "return" line for the heater core. The lower hose off the engine to the heater core is the out flow. If you are returning to the radiator though, the line under the thermostat should flow coolant.
Why not run the oil feed under the engine? Or between the thermostat housing and valve cover?
So from what you've said, this image of a V8 should have the same flow of coolant right? So putting a tap in the part of the thermostat housing under the thermostat (the part that connects to the head) should be a pressure line and the hoses feeding and exiting should be lower pressure since they're after the thermostat and are being pulled in by the radiator correct?
Your comment about the the line under the thermostat being a return line seems contrary to the diagram and the fact that coolant would flow into the water jacket, out of the head, and push against the thermostat. It seems like the heater core line that goes into the water pump would be the one "returning" coolant from the heater core to push into the block
69 Buick Special Deluxe. Intercooled Turbo Chevy 250 @ 15psi on a stock long block. It's kinda fast.
