loss of power while driving

72 Duster. New factory front and engine harness. Have a 518 OD trans. Car ran great for 75 miles today and did this after sitting for an hour at the store. I just cycled the ignition to start it again. Started right up.

Some more car details.
New 360
roller cam
aluminum heads
street demon carb
Orange box
518 Trans
3.90 gears

Got about 300 miles on the motor and 100 on the trans.