well, typically an H-pipe fits a fox body stang (87-93) and an X-pipe fits the SN-95 stangs (94+). Now, keep in mind, that is the most typical setups. You can have a custom one made.
An off-road H or X pipe simply means that is does not have cats on it. This is illegal because of the emissions crap.
An off-road H-pipe cost about 150-200 and the H-pipe with cats will run you about 300. Make sure you get High-Flow cats. They are still restrictive but they are must better then the stock ones.
You should wait and get the H-pipe (my 2 cents). If your going to bolt it up, it will be easier because you already have the pipes apart. If you are going to weld it, most defently because you don't want to have to re-weld it.
To give you some of the characteristics of the off-road H-pipe... You will hear little backfires (I like it), If you run a little rich you will smoke a little out the back at WOT (like me
), your exhaust will cool off faster, gives you a deeper richer sound, and you will have more HP/Torque.
If you don't have emissions hook ups, I wouldn't suggest it. If you do... go for it. If you don't and you have extra money to burn, buy both H-pipes (with and without cats) and when time come to get inspected... just swap pipes for the time being. (I do that
)