I fully understand what you mean about the glass packs on your vehicle. my play toy is a 87' camaro with a 385cid that also has full length 1 3/4" primary headers going into 3" pipes but i dont have glass packs i have the dynomax 16.5"x4" bullets, that runs 11.20's and yes it certainly is loud. I wouldn't not want my truck to be that loud, but i think when you combine the resonator, catalytic converter, and either the glass pack or the thrush welded muffler it will muffle a heck of a ton more than just glass packs sitting on straight pipe.
you mentioned you wanted to put flowmaster 40's on your vehicle. you should do some research on how the flowmasters acutally flow vs some of the other brands. you will find that they are next to the bottom of the list when it comes to CFM. now i dont believe in paying 100+ for a single muffler like you would with Magnaflow, but dynomax (who makes thrush) makes mufflers extremely similar the 40 has but yet acutally flows much better and cost about $20-30.00 cheaper.
the thrush welded, and the dynomax ultra flow welded