On my 426 stroker I have long tube headers, 3 inch pipes all the way, with two resonators before the mufflers, 2 borla x mufflers and 2 resonators after the mufflers. It is still a little a bit loud with the stock insulation. On the outside it sounds awesome but inside I would like it a little bit quieter. If I was to do it again I would go with the dynomax turbo but the opposite side in/out so its the real design and not the split chamber one.
The gear ratio you mention for the diff will not allow yo to go low rpms in the highway, neet a much taller gear.