[68gmsee]

Backfires through the exhaust or carb?

Since it's only been a little over a month since it was parked and if truck was run frequently prior to the hurricane, I doubt the gas is the problem. There's several things that can cause backfiring so start with the basics.

First, use a timing light and set the timing to specs then start trouble shooting from there. Check for vacuum leaks, bad ignition parts like wiring and spark plugs. If you have HEI check distributor cap and rotor and make sure the ground strap under the coil is not rusted. Moisture can do that if it's been very wet where the truck sat and it's easy to check it.