Ultimatly if you really like Mustangs and want to go all out and get good mileage with good reliability do small stuff to the Fox and get yourself a 03/04 Terminator. Their whole driveline and block can handle 650-700 rwhp with minor upgrades like half shafts, clutch and a good tune. Fox guys please dont flame me for that we all know its true and cheeper than doing everything I said earlier added with bently429 which is all the stuff I forgot. You can find a termi for 17-18K (high mileage) and everything me and bently said + the price of the fox is way over 20K because no way are you gonna be able to do all the work we suggested. The Termi on the other hand only needs a better S/C, exhaust, intake, fuel system and a tune which you can easily do yourself except for the custom tune.
just my .02
just my .02