I too left my OEM overflow up front in the stock location. We basically have two major high points in the car, assuming your front to rear transfer tubes run toward the bottom of the frame. The top of the radiator is a good two feet above my transfer tubes and the top of the engine is again a good 2 plus feet. I opted for a 16lb cap on the radiator and a 20lc cap on the pressurized reservoir on top of the engine. I get good circulation and any major air bubble up front vent first to the overflow. In theory, the pressurized reservoir cap should never unseat and vent. After filling and purging the system I just check the level in the overflow and adjust as necessary (it raises and lowers as one would expect with thermal expansion) and the rear cap I top off as air makes it up there and the level drops. I don't have the most mileage on the setup, but do have an entire day at VIR's track on it without any cooling issues.

On a side note, I ditched every bit of that corrugated flexible metal tube FFR provides. Getting the air out of that stuff is essentially impossible. Every corrugation holds a large bubble times a billion corrugations. Overtime it will come out I suppose, but I do not have the patience for that. They are hard to seal as well. I found some long rubber tubes for like $9 bucks at Rock Auto and cutting sections out of them with the proper bends and lengths was surprisingly easy. Different portions of one part number basically handled all four corners, think I only have 1 or maybe two couplers up front for the long intersing journey from the foot wells to the and bottom of the radiator.. Could dig up the part number if anyone is interested.