It sounds like you've got more money than most of us on here do at the time. Not that there's anything wrong with that, it just puts you in a more exclusive/less trafficed area of these discussions. Most of us have D21 hardbody pickups. You've got a D40 I think.
Sound insulation can be a great idea with a loud/boomy exhaust, the resonance can be unbearable sometimes, and delightful at others.
Did you have the I-5 or I-4 Colorado? I test drove an I-5 and I thought it was peppy and powerful, at least compared to my truck.
Are you trying to lift or build this thing for hardcore off-roading? If so, I think
http://www.nissanoffroad.net is the authority on that one.