My ears are still ringing

[vectorit]

I just got my 1970 K10 back from getting a new exhaust system today, and went out for a spin to enjoy the latest upgrade to be done to the truck. The system is OE ram manifolds, dual exhaust with H pipe, and hemi turbo mufflers exiting behind the rear tires.

Truck sounded great! I could hear the engine, and everything else going on in and out of the truck. So I went for a little scenic route to get up to highway speeds, then pulled over to snap a couple pictures while the truck idled for a few minutes.
Hopped back in the truck, put her in gear and took off with a little bit of zip. As soon as I took my foot off the gas to shift... BANG! It sounded like a rifle shot fired from my cab out the window. Engine still running along, so I shifted into 4th and tooled along wondering what the hell could have caused that.
Then just as my ears are back to adjusting to some what normal noises, my exhaust sounded different...

I drove all the way home wondering what the hell let loose.

Well, as you can see both new mufflers with less than an hours worth of drive time on them are blown out and now have to be replaced.

Any ideas on what could be the cause of this?
I am suspecting my Pertronix II ignition may have caused this, so that is the first thing I am going to investigate tomorrow.

The truck runs great, and has never back fired like that since I have owned it.