Pre-and Post- thermostat. Pre-Thermostat would be considered pressure (between the waterpump and thermostat). It is a bit of fuzzy math, particularly when the thermostat is closed and flow is through the by-pass hose.
That coolant drain plug would flow coolant, but is pretty small. What size line do you need? Just as important, where will you return the line? Your best bet maybe to tee into the heater hoses. The lower one off the pump is the higher pressure.
Thanks for the info. I'll nix the coolant drain idea then. So it seems like putting a coolant tap for the feed line under the thermostat in the thermostat housing would work? This would be on the same part that would have the feed line to the heater core. I'll probably use a bulkhead fitting on the radiator hose to the water pump inlet for the return. The feed and return barbs on the coolant banjo fittings I just ordered for the turbo are 3/8" so I'll jusy use 6AN lines to match. Should be more than enough.
Just for reference, the oil feed is a 4AN fitting and the oil return is a 10AN. Wish there was an oil feed line on the driver side of the car so I didn't have to run it up and over the valve cover but we work with what we got.
69 Buick Special Deluxe. Intercooled Turbo Chevy 250 @ 15psi on a stock long block. It's kinda fast.
