You need some real mufflers. Glasspacks are straight-through design, and it's not uncommon to get a backfire on deceleration, particularly if the mixture is leaning out a little as the throttle is closed on a coasting situation.

I like to use Cadillac mufflers from the mid-70's. The inlet and outlet pipes are both 2 1/2" diameter, and one muffler will handle the volume requirements of a 500 cubic inch engine. Dual pipes with those mufflers installed have virtually no back pressure, and they're QUIET!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!