360 Backfires Under Load

Did you check the pumpshot? With the car not running look down in the carb and crack the throttle by hand and see if you get an instant shot of gas. If so then double check your timing. A good place to start is around where you had it at 15* with the vacuum advance unhooked and plugged. Once you have that right check your vacuum at idle using full manifold vacuum, rotate the distributor until you have the highest reading and back it off a notch as Crackedback said. Double check your timing to see what you end up with. You can then move onto adjusting the carb.

Timing then carb.