Make sure both screws are turned out equally, from what i've read in the past. Granted it's main job is for only the idle, but that's a start. I've had mine not set the same before and it'd pop through the carb if i tapped the throttle from an idle. Make sure you're timing is set right, should be about 36 (without vacuum advance) for total.

I'd go with the mufflers I suggested above, the thrush welded, as they are identical in every way to a flowmaster original 40 series muffler. Sound great and aren't as expensive. Look on youtube and see what you like.

 

Dark: We have the same exact truck! Like NW said above, timing is important, probably a good time to get out the timing light. Also, what are you running for ignition and advance currently? Stock engine/cam/compression? On my 390 I'm running the stock points dissy w/10° initial advance and the truck seems to be running fine.

 

A carb tuned a bit rich, along with either an issue in the ignition system and/or an exhaust leak = huge fireball out the exhaust on decel.
I'd suggest giving the rest of the exhaust system a good inspection, those pics make it look to be rather old, so rust holes wouldn't be unheard of. The mufflers were already starting to rust thru, so the rest can't be too far behind.

 

It's pretty much all stock minus the Edelbrock carb and intake. The previous owner said it had an RV cam, but I'm not sure if I believe him. He said the engine was recently rebuilt, but it's got far too many issues to have been rebuilt; he also said the tranny had been rebuilt and had a manual valvebody installed, yet it shifts perfectly fine on its own and has a good sized leak from the pump seal at the front of the tranny...
When we did the intake, my brother hit it with a timing light and got it pretty good.

I'm thinking of taking it to a shop to have the carb jetted and having them check/adjust the timing anyways though.
The pipes are pretty rust coated, but just surface wise. The truck has been in SoCal for all or most of it's life. However, I would guess that the exhaust is almost as old as me, if not older....I'm 29.

I'm trying to remember exactly when the fireball occurred. It was popping and bucking so I gave it some gas thinking I could get it smooth out and it popped harder and louder, and then came the boom. I may have lifted as a result of the increased bucking right before the muffler went. Mufflerless, it still pops and bucks like before, but now at lower rpm. Used to 60ish in 3rd, now it's 50-55ish.

 

New distributor cap and ignition coil installed, took the truck all the way to 65 and no symptoms of the problem. I declare it safe to reinstall an exhaust system. I'll still see about getting the carb jetted and tuned, but I'm just happy it's not bucking and freaking out anymore.

 

Sounds great, glad you got her sorted out! Be sure to shoot us an update when you get mufflers back on it

Ignition can be a real pain in the butt on these trucks, and its so important to have everything cleaned up and in good shape. I bought a PerTronix Ignitor and matching coil to eliminate my points, but I never installed it. I did too much reading here on FTE and decided to go DuraSparkII instead. I'm lucky in that I'm the 2nd owner on mine, and my grandfather was the PO. Most of my automotive mechanical knowledge came from him AND this truck so I have a pretty good idea of whats been done to it in the past.