This link is what I used as a guide during the Mac True Fit longtube header in stall on my '02 Mustang GT.  Its a pretty good guide for what I did other than he installed BBK longtubes, which require a little more work:  http://www.fnsweet.com/garage/lt_install/index.shtml

Here are some tips to help insure you don't take as long as I did to get it done. 

1.  Don't drink too much beer and don't work too long straight!  Near the end I made some really boneheaded mistakes that cost me alot of time.

2.  The install guide is for BBk, if your using Mac True Fit longtubes you don't have to do as much work to get them on.  For example, because the Mac's tubes aren't connected to each other at the manifold end, you don't have to worry with removing the dipstick tube or loosening the AC compressor etc.

3.  Mac may be easier and less expensive, but BBK has a better coating and will look better after time goes by.  I chose Mac because I got a great deal on a used setup.

4.  Allow 10-12 hours to do this job, 8 if you have air tools.  It took me 12 because of a few boneheaded mistakes, but it should have taken me 7-8 (I am pretty fast and I have some electric tools, like impact wrenches etc.)

5.  Three hours were lost because I stripped the two rear bolt holes on the driver's side heads.  This was because the studs came out (no big deal in itself) and the smog tube was pushing the rear tube out of alignment.  I got too far on both before I realized it.  Those were the last bolt to finish that side (dammit!).  It tool 20 minutes to get everything off and realized I stripped them. 2 hours of driving around looking for a thread chaser (to no avail by the way), so I had to use the stock studs and run them up and down over and over again through the stripped holes, finally cleaning them all up.  I then installed the studs first this time and all was good (3 hours later).

6.  After getting tired and nearing completion, I carelessly put together the suspension and components on each side with the caliper hung on teh opposite side.  By the time I realized it I had to remove everything from the sides, move the caliper, and put everything back... Im still cussing at that one.

7.  Stock spring installation requires either a spring compresser (recommended but I didn't have) or a friend holding the a-arm to the ground with the car still jacked up, and myself pushing the spring in at the bottom with a breaker bar.  Once it was stable I had to jack up the a-arm to compress the spring and get the shock bolts in.  Major PITA.

8.  Make sure you do it right the first time, especially bolting the headers to the head.  Tightent them down the right way so you won't find yourself in a position to have to do it again.

9.  I haven't bolted the cat-back on yet so I don't know how it sounds yet.  I will do it tonight and let you know how it comes out!

10.  Be safe and listen to good music!

Well thats all I can think of right now, but if I think of anything else I will update this.  Remember, longtubes are usually good for 20+ more hp than shorty headers, so its definately worth it.  Unfortunately its a PITA to install them and any shop is gonna charge you $500 + to install them.  Add that to the cost of the headers and new H-pipe and your looking at spending over 1k.  Be sure that you have the time to install + cost of parts, or over 1k if your gonna pay someone to do it... Its prolly not the best choice to do this before gears or a cat-back system due to the cost vs performance increase, although I did it before gears.

Geez thats a long post lol.  If anyone has any questions or tips then post away.

Ok, I finished up the h-pipe and cat-back exhaust last night.  And the results are:

Wow, the power increase is unreal!  I expected it to be stronger but damn!  I would say I gained 30+ hp at least from the full exhaust.  Once again I used Mac True FIt Long Tubes / Mac Pro Chamber / Magnaflow Cat-back exhaust.

Pros: Ungodly power compared to stock.  Engine now pulls hard all the way through redline instead of falling off after 5k.  Gnarly exhaust tone, very deep compared to the stock exhaust.  No resonance!  Interstate driving is barely louder than stock!

Cons: Not as loud as I wanted.  You would think with longtubes, no cats, 2 1/2in exhaust and straght through mufflers it would be louder.  That prochamber may add some ponies compared to h-pipes, but it damn sure quites the whole thing down!  My wife liked it though...  Also a big con is the installation. OMG what a PITA!  Well worth the power increase though.  IF I had to do it again I would.