I purchased flowmaster's AMERICAN THUNDER kit for my 1995 F-150 XLT 4x4 a few months ago, and I am very happy with it after a few changes. Flowmaster makes 2 kits for the truck, one is the FORCE II, the other is the American Thunder(AT). The AT is supposed to be extremely loud, and much louder than the force II. After recieveing the kit I was surprised that it came with a 50 series delta flow muffler. Now, the 50 series is their mild muffler, and delta flow is a new technology that makes the muffler resonate less so it wont be heard inside the vehicle. Nevertheless I installed the kit and was rather dissapointed with the sound. While driving the truck I could barely tell any diffence in the truck sound (I dont know about you but i want to hear the beast growl when I punch it!) from the rear of the vehicle a difference could be found, but not what I was hoping for. So I purchased a Flowmaster 2 chamber 40 series muffler (NON DELTA FLOW), this is the muffler that I assumed the kit would come with. Since this muffler was about 4" shorter than the 50 series, i needed to extend the 3" inlet pipe that I had installed when I put the kit on. However it was impossible to find a car part store with 3" exhaust piping. so instead I had to extend both of the 2.5" exit pipes. Once I started up the truck I was happy. Much louder sound from the truck that can be heard easily from inside the truck. My overall opinion of the kit is a good one, it came with very nice stainless steel tips, and everything fit together well and installation was simple. However if you are looking for a loud, agressive exhaust, plan on purchasing a 40 series muffler along with your kit.
-John

1995 F-150 XLT 4x4, 31"x10.5" Goodyear MT/R, K&N, Flowmaster, Rhino Liner, 8" Pro-comp lights, soon (6" Skyjacker class II lift, 35x12.5 swampers, front locker)