Contuned backfire issue

Welcome to 2CarPros. Is it backfiring out of the exhaust or carburetor? What ignition set up are you using?

mostly exhaust, sometimes intake. duraspark 2.

Adjusted valves yesterday, I think I still have one intake too tight, because it's mostly backfiring intake. Have fast idle set to 1,800, roughly. set distributor to number 1 TDC. Balancer is indexed correctly , will sort of start just around 500 rpm then shuts off, or backfires and shuts off.

Thanks for all the additional information. Have both 351 firing orders been tried?

Just some general thoughts.
Backfiring indicates the fuel is combusting at the wrong time so we have to be looking at some sort of ignition issue. You said you changed a distributor. Did that include the cap and rotor? Those would be my priority suspects along with cross-firing wires or a bad coil. The primary side sounds all brand new so I would just confirm the power supply isn't being interrupted.

Take valve covers off and see if all are working. it may be a worn out cam or lifter bad or both. also check for a bent push-rod.

Sounds like a timing issue.

In order for this to be a valve problem, it would have to be a continuous miss, not random. It also would have failed the compression test and likely made noise if valves were the issue. With an ignition timing problem, the backfire would be under acceleration only. If it is a completely random popping, that would have to be ignition. You never really described the type of backfire you are experiencing.

Thanks for all the input. As stated just did valve adjust, and got worse, did the adjust per firing order,(covers off, plugs out) engine only has 2,100 miles on it. block casting below. All started about seven months ago, with a major backfire from exhaust. flash of light so bright it reflected off a white wall as i pulled up to a stop sign. Just left work 05:00. Denver. Truck had been flawless for eighteen months. seems like I've made matters worse with every attempt. I do not know why it happened, no amount of checking has lead to cause of first backfire, nor any repair has changed anything for the better, only gotten worse. Now I sound like I'm whining

You changed dizzy and wires, what about plugs?

yes new plugs, also just installed new BWD OEM coil and connector
Two kinds of backfire out of exhaust. #1 is hugh single bang like a gun shot.
The other is more like a rumble, very low rpm, will not come up. and dies, restart same rumble, low idle, shuts off, restart third time dies right away, hugh backfire. All out exhaust.
Static timing to number one TDC vac advance plugged, sometime starts and runs up to 2500, set total timing to 34 degrees , let it idle and the backfire starts.
When i was still able to drive the truck the huge gunshot backfire was on deceleration.

Okay, to be sure I have it straight it was rebuilt and running fine then the big backfire and it hasn't run without backfiring since. You rebuilt the carburetor, changed out the ignition system and still no go. I'm thinking the backfire may have been excess fuel getting into the exhaust system and igniting. Maybe drop the exhaust and see if things improve. What has been changed from stock on this engine?