Engine keeps backfiring?

5.2L is a 318 c.I. 360 is a 5.7L. Doesn't matter though, they look identical and are timed the same way. Do you have a vacuum hose going to the distributor? If so it must be disconnected and plugged. Seems to me you should have a computer taking care of the ignition timing advance. Set me straight on whether there's a computer on the truck, and whether you have a vacuum advance unit on the side of the distributor, but first double-check the firing order.

Backfiring is usually caused by having two plug wires mixed up. The distributor turns clockwise and the firing order is 1-8-4-3-6-5-7-2. Front to rear on the driver's side is 1-3-5-7 and the passenger side 2-4-6-8.

If the backfiring started on its own one day it's more likely you have a leaking intake valve. Re-timing it isn't going to solve that.