glass pack and and muscle car sound

glasspacks that dont kneck down in the middle cant be anymore restrictive then a chambered muffler. I have 18 inch flo-pro glass packs that didnt have any inlets or outlets and they are 3 inch diameter inside, the exhaust guy welded on the inlets and outlets to fit my 2 1/4 inch exhaust pipe.
I dont see how an 18 inch glass pack with a 3 inch core diameter could be restrictive at all.