i bought my first barracuda 340 fastback s 4 speed back in high school (69-72). the plymouth dealership mechanic who i bought the car from and who had bought the car new had installed hooker fenderwell headers. that car had a holley 750dp sitting on an edlebroc alluminum highrise. otherwise the 340 was stock with a the factory duel point distributor, x heads and the 4 speed factory cam. the car had 3:91s in the diff. the best time the mechanic guy turned with the car was a 12.29 with open headers and slicks. i only disconnected the exhaust pipes once from the collectors while the car was in my driveway and it was VERY loud!! if you are going to drag your car and run the headers open then i'm pretty sure you will get at least a 5-10 hp increase with fenderwell headers over under chassi types. fenderwell headers flow better than under chassi. however, if you are just building a street car, you are never going to disconnect the headers from the exhaust pipes. there is no doubt that chopping up the fender liners don't really add anything to the looks of a car and unless you're trying to win your class at the track, in my opinion, it's not worth cutting up the car for fenderwells over chassi. also, i will tell you that i had to be careful sometimes when i drove my first cuda in the rain not to drive through too much water. with fenderwell headers, you can get a lot of water, dirt and mud splashing up into the engine bay. the headers on my first cuda were painted bright hemi orange and they did look cool when my "fish" was lined up in the back row of the local burger joint. there's no doubt that fenderwell headers will make a street car look like a car that you probably won't beat in a red lite to red lite run.