Really? You don't think that the performance of the GT-R or GT350 could be bettered by someone building something based on a used car? Who said anything about starting with a new 5.0L Mustang? After all, Sage has a '12 model Raptor that he'd be starting with in the original example. I think that bettering the performance of a GT350 would be absurdly easy if you started with a 1-2 year old 5.0L Mustang. A blower setup, big brakes, and some high end shocks/suspension would get you there... and leave you plenty of budget for stuff like a cage to stiffen the chassis and give you a place to attach proper 5-point harnesses.
Anything is possible and can be done "cheaply". Kids put 1,000 hp in $2,000 Civics. Just because you can, doesn't mean it is a good idea. The resale value of all those upgrades on an old truck is about the same as burning money in your driveway. But yes, if you want to flog a truck in the desert without a care in the world, a tuned beater setup is the way to go and simply treat it as a disposable vehicle.