Backfiring on Deceleration...

[70 Pony]

I'm a new guy so be gentle....Running a 351 W in my 70 Sportroof. 30 over truck block, TFS heads< Crane Roller rockers, Comp extreme energy cam 268 506 512 lift, EBrock Perf RPM intake, Holley 650 DP, Fed Mogul Hyperutectic Flat Top Pistons. Mallory Dist, MSD Blaster ignition and coil.
My exhaust is Hedman Headers, leading into 2 1/2 " pipes with Dyno Mac Race Mag Bullet Muffs, 4" X 9", dumping out in front of rear wheels....Now I've only been to the track once and ran a 13.44 at 107 MPH...the 3.55 Posi kills my launch and that was on New street tires BFG TA's as my grag radials were showing steel and I knew I wouldn't pass tech....
OK Now the question...On acceleration the car runs great, no misses or backfiring....but when I release the accelerator I get loud (almost gunshot) backfiring from my Pass Side exhaust..had a cross pipe welded on...same thing...Now I'm probably running a max of 6' of exhaust tubing from the collectors to the tips, and no that this can cause unburnt fuel igniting as it hits the hot pipes and cool air..BUT it's only doing it on the passenger side...went to one step colder plugs...no help...the only thing that lessens it but does not eliminate it is advancing my timing. My timing (initial)is near 27 without pinging or sluggish starting even when hot...But I know this is way too high...but if I back it down the backfiring gets worse...Seems to only do this when I suddenly decelerate at over 4 Grand..I can wind it to 6 grand and then the back firing gets pretty loud...maybe about 4 or 5 gunshots is it.....
Bored you long enough...any suggestions...
By the way..the 650 DP is jetted at 78 and 69 with 37 squirters. THANKS>>>

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Eddie D, 70 RestoMod, Fastback, 351W, Arrest Me Red.