1st off, this is the boats and boating subsection of the cars and transportation section.
you need to re ask your question, and you want the ford subsection of C and T, which is under car makes.
2nd off, what anybody will tell you there, forget trying to get more power out of the inline 6 and drop a small block v8 in there instead.
you'll get a lot more bang for your buck by just getting a v8, and that v8 has the potential to be way more powerful then that straight 6 could ever be.
inline 6 motors are heavy for the amount of cylinders. your suspesion will be able to handle the weight of small block v8 with no problems. a big block however would require a rebuilding of the suspension.
your also probably going to need to swap trannies too with the motor change....
which brings me to a suggestion on where to get those, that will give you great power and improved fuel effeciency at the same time.
find yourself a newer truck that was totalled out but only has body damage (motor and tranny untouched). probably one that got rear ended.
chevy, ford, dodge, as long as its got a v8 and a 4 speed tranny, its capable of delivering about 300 hp and nearly 20 mpg as is. aerodynaimcs, size and weight, you might get better mileage then the truck with the same motor in a little mustang.....
ford is what most people would choose with a ford, but there are plenty of powered by chevy older mustangs on the road too.
and the chevy trucks outlast the fords and the dodges......