engine backfiring thru carburetor

Reset the timing. It is probably too far advanced.

Is this a replacement distributor? Originally your truck had a conventional ignition system (points and condenser)? Jim

This is the original distributor, although the magnetic pickup setup is "only" 25 yrs old! And i will try resetting the timing I just didn't think about it since nothing moved

Another cause of backfiring through the carburetor is a lean air to fuel mixture. Especially if it's backfiring on acceleration. Possibly bad accelerator pump in the carburetor or clogged jets. Remove and inspect the spark plugs. If the tips are white you have a lean air to fuel mixture. Hope that helps! Jim