Did it run OK before you put on the new distributor?
Getting the timing right is the first this to do: is there a hole on the front face of the distributor side of the clutch housing? If not, what type of clutch housing do you have?
Figure 6 show the location of the timing hole/pointer/ball. There are a variety of ways you could "dial in" the timing: set it to the highest vacuum reading, set it to the highest idle rpm, and/or set it to best acceleration/deceleration by progressive adjustment.
Progressive adjustments means: rotate the distributor slightly in one direction and drive the truck. If deceleration/acceleration gets worse, rotate the distributor slightly in the opposite direction; if the deceleration/acceleration get better, continue to slightly rotate the distributor in the same direction until acceleration/deceleration feels good. This is a lo-tech technique but it should get rid of the back firing (if it is due to bad timing).
The backfiring upon deceleration is an indication/result of unburnt gas. This could result from a variety of things, including bad timing, bad valve adjustment, incorrect firing order, or something wrong with your carburetor (bad float setting, for example).
