On my old L88 powered 68, I had Dynomax Ultraflow's on there (3"). Those sounded good, and in period if that makes sense. I chose those in that particular application, as the regular Dynomax turbo mufflers would neck down to 2.5" inside. Given you are going with a 2.5" system, I would suggest this particular Dynomax muffler. It packages well in an F body car. If memory serves me right, this is true to the old Corvair turbo muffler.
https://www.summitracing.com/parts/WLK-17733
Hope that helps,
Tim