The glass-packs I had on my Chevelle a long while back were just a straight through design with a 4" OD and a 2.5" straight shot through the main body. The 2.5" 'tube' was perforated with scallops that penetrated the 2.5" ID by about an sixteenth of an inch. These scallops were open towards the engine side of the exhaust and directed some of the exhaust flow into the fiberGLASS filling between the ID & OD. Here'* a pic I found online:
You really won't gain anything by hitting the outside of the glass-pack as the sound deadening material is just fiberglass packed between the outer shell and the perforated inner liner. The reason why they get louder over time is because the more you use your car, the more fiberglass gets broken up and blown out the tail pipe until sooner or later, you've got nothing in there to deaden the sound.
Like I said, I had them on my Chevelle. My parents could hear that thing coming from over 5 miles away on a clear day. Once I got a warning for excessive noise, I switched to Thrush Turbo'* and will NEVER go back to glasspacks again. The exhaust note from the turbo'* was amazing and didn't have any of the popping on deceleration like the glasspacks did. There was also enough bass to set off car alarms at my high school when I'd stab the throttle clear the carb before shutting it down!!!
My advice, unless you want your car to be loud and obnoxious, DO NOT get glasspacks.