Well, it's my turn to ask for suggestions..

Ima sticking to exhaust or springs, or both. With no tailpipes, checking the exhaust is easy. The louvers in the Gpacz , need to be facing in the correct direction. The muffler are usually marked in and out. But the louvers by themselves may not be to blame, if the muffs are correctly sized.
We used to put one size bigger ones on backwards on purpose. They were a bit louder, but with the louvers pointing backwards we were fast. Or so we thought... Heehee.
Naw, the big thing with Gpax was they tended to puke their packing. I think the ones with holes instead of louvers were better at keeping their packing,packed; but they were noisier too. Shine a lite in them;easy check, or drop them and let her rip!
BTW, I don't suppose you stuffed the headpipes into the glasspacks, and muffler clamped them too close to the ends, crushing the headspipes. Naw, I didn't think so.