Lot's of popping and cracking on deceleration...

[71C10SWB]

I've got my '71 swb back on the road and I like everything except all the noise it makes when I'm decelerating.
I get a lot of cackling, popping and backfiring, especially going down a big hill.

Before I give you the set-up, I want to say, I didn't set this up.
It has:
350, Powerglide, 6-cylinder gears, hooker headers, single plane Weiand intake (high rise), Edelbrock carb (cfm unknown), glass packs and full length 2.5" exhuast.

Is it the glass-packs, the engine set-up or just the nature of the beast?

What would help? Turbo mufflers? Flowmaster mufflers? Crossover pipe?
could timing have anything to do with it? I've put a light on this thing, who knows where it's set at. I've not been really impressed with the carb either (it might just need a rebuild.)

Any feedback would help.