383 woes

It will backfire at higher rpm when moving or just winging the carb. Didn't used to do it, in fact it ran great. If I bring up the rpm's slowly it's fine, but try and spin it up quick and it pops. The only thing I changed before this started was new plugs and changed out the Thrush glass packs for Thrush welded mufflers (the wife was complaining about the interior noise). It has headers with 2-1/4" outlets, I'm running 2-1/4" pipe all the way out. The exhaust dumps out in front of the rear wheel wells. I've tried timing anywhere from 0 degrees to off the scale with vacuum advance unhooked and plugged. Stock spec is 2.5 BTDC +/- 2.5. I'm wondering if the intermediate shaft is off one tooth. I've set the idle mixture by vacuum but it seems to idle more smoothly if it's leaned up a little.