Old Navy,
The Kansas Kustom manifolds are well made and reasonably priced. Smithy's blue mufflers come in 3 lengths. In my opinion the middle length muffler sounds the best, 30's are too long for the traditional sound. You will also get different Smith's sounds if you turn the mufflers around. If you want the triditional 6 cyl rap sound do not install a crossover. If you want it quieter use a cross over. The best place to put in a crossover is just past the crossmember where the brake line crosses over. I've also noticed that the cross over will help hold up the pipes if you ever have to take the Manifolds off. On your question about gas mileage: watch the size of your pipes from the manifolds to the mufflers, 2" or 2+1/2 inch pipe will give you less backpressure and also less mileage. Your engine needs a little backpressure. Its hard to find but look for nothing larger than 1+5/8 or 7/8th from the manifolds to the mufflers. From the muffler back you can use 2" or larger. Some research and google search under "header design" will give you some more background. Have fun.
