It would probably cheaper to buy a cat-back kit than to buy the flow-tubes, mufflers, and tailpipes seperately. Flowmaster sells the mufflers and tailpipes seperately, but they don't sell flowtubes seperately. Most muffler shop can make you the flowtubes and weld the mufflers to them, but you would have to go around to your local shops and get some quotes. A cat-back kit would probably be cheaper than that route anyway. Although welding is better, most cat-back kits slip-fit and clamp together, so you could feasably install one yourself at home. If I were you, unless your flowtubes and/or mufflers have actual holes in them, I'd just clamp the old flowtubes to your new H-pipe and see how it does. If it's so rusty that there are leaks, then something new is probably in order.
Also be aware that the Flowmaster American Thunder kits now come with Delta-Flow (slightly quieter) mufflers ONLY. None of their kits come with the original 40 series mufflers anymore. If you want the original 40's, you'll have to order the mufflers seperately and then order flowtubes and tailpipes. Mustangs Unlimited (Ga.) has flowtubes at very reasonable prices. Flowmaster mufflers and tailpipes you can get anywhere. Also, if you have stock flowtubes and/or tailpipes, they are 2.25" in diameter. Most cat-back kits and aftermarket exhaust parts are going to be 2.5", which will give your whole exhaust better flow and a little more sound.
Hope this helps!
I figured that's what you meant.
