glasspacks
 

Anybody know what size glasspacks I should get for my 07 4.0 with custum 2.5 inch duals?

And would they be mounted in the very back or somewhere in the middle?

You answered your own question. If you run 2.5" duals then you need 2.5" mufflers or in your case glass packs.

how would glasspacks with duals sounds on a mustang? my dad had some, it was single exhaust, but it sounded rice as fuck, pretty lame

I have em. They're deep and clean through 2800 and above that they get poppy. I'm getting mac longtubes and flowmaster 44s

well he could be talking about the length of the muffler, glass packs do come in many lengths.