The 5 liter in 1990 was rated at 225 hp and 300 lb ft of torque. My 1992 GT convertible with automatic got 25 mpg on the highway and about 19 in town.
The Ford racing 4 wheel disk kit may have been discontinued. I know that FR stopped making many 5 liter Mustang parts a few years ago. There are plenty of places that sell 4 wheel disk kits. www.50resto.com
is a good place for 5 liter Mustang stuff including 4 wheel disk.
There is so much stuff you could do that would get more power. Some guys do superchargers and others do 347 stroker kits. Cheap mods, start by removing the air sliencer and getting a good cat back exhaust system.
Go to a junkyard and get the GT-40 intake and heads off a 1996 and up Explorer or Mountineer with the 5.0. All 5 liters from 1996 and up in these trucks had the GT-40 stuff. This will bolt on at least 30 hp. A mild cam is good for another 40. You want to do a set of rear end gears too, something over 3:55. If you have an automatic, consider a 5-speed swap to really wake up the car.