Re: putting in a v8
welcome semperfi! glad to see you chose to pick a mustang over the cars that you could have chosen...
As far as mods, what I'd recommend you do is check out the 3.8 section of the board, and try to learn as much as you can from lurking, and researching about different setups and such to see what combo will get you whatever you desire in the car. Those guys (least the one that I know) tend to be reaaalllly helpful, and will point you in a direction if need be.
Welcome again, and thanks for your dedication to your country, and the freedoms of those who live in it!