66 Dart, 408 runs hot. HELP PLEASE!

Pitch that 180° thermostat for a high flow 160°, and get rid of the bypass. COMPLETELY.
Just stick a 3/4 pipe plug in the manifold, and tap the pump for 1/2 NPT and stick a plug in that too. NOW 100% of your heated water HAS to go through the radiator. Recirculating heated water is fine for grandma's scamp, in the snow. But is has no place on a hot street car.
Take it to the bank, I've been doing this for 20 yrs.
The pump can be modified too, but I don't know that I would recommend it on a crank driven pump..

BTW, you'll prob run 2-3 tenths quicker if you get your starting line temp down to about 140°.