I'll poke at this one only because it happened to my Dart once...My water temp was hitting 220 down the road...weird for a Slant 6 to have that happen. Everything was in tip top shape. I checked my Trans dipstick and I was a little low. I topped off the trans and got back to business as usual. I can only assume that the trans overheats and since it is aluminum and mated through a bell housing, you are heating up the coolant from the transmission side.

Have you run an A/F meter on it? make sure you have the right numbers through your powerband. The idle circuit is pointless...our cars don't sit at idle most of the time... you have to get out there to test and tune.

Not to hijack the thread but I have a 440, AC pump, factory cast iron housing and cast pump. 10:1, Eddy heads, Dual 500cfm Carbs. 26inch 4 core radiator, tranny cooler, 7-blade with viscous, 180º Napa stat (Mr Gasket are trash, bad QA). I even threw in Water Wetter. I know I have some test and tune because I just got my RPM heads on but I hit 180º driving down the road and 195º in traffic.

I have no vacuum advance. Its a mechanical dizzy. Are you sure your dizzy is setup correctly?