Where would I pull the source and return lines?
Ayuh,...... I donno where the pipe plugs are on a Ford,.....
Ya want pressurize hot water from the intake, before the t-stat, goin' to the heater, 'n yer return goes to the circulatin' water pump housin', or "T" into the big hose feedin' the circulatin' water pump, aka: the low pressure side,.....
The Chevy motors that I work on have a 1/2" pipe plug in the intake manifold, just before the t-stat housin', 'n a 1/2" pipe plug in the circulatin' water pump housin',.....
An accessible pipe plug in the head would be a good feed line, or even a block drain as AD's is,.....
Heat rises, so closest the t-stat is
Best,.....