Fifth gear is the easiest to get too. All you have to do is disconnect the tailhouse, and it hangs out the back of the case. You punch out one roll pin on the shifting mechanism, and it slides right off the shaft as an assembly. Piece of cake. You have to take the mainshaft to a gear shop to have the new gears pressed on.
In order to put a TKO-600 in your car, you are going to need a driveshaft extender. Steeda sells it, and it looks like a mini wheel spacer. The TKO-600 comes with it's own special bellhousing, you don't use one off a Fox. However, you do need a clutch fork from a Fox. If you intend to keep a dustcover for the fork, that also has to come from a Fox. Throwout bearing is the same. You will need to drill the holes on the crossmember back a little. Also, the factory hanger for the mid pipe won't fit. You will either need to leave it off, or build your own hanger for the mid pipe. I made a special one out of quarter inch steel, washers, and two S hooks from home depot. The headers shouldn't be an issue. An SN bellhousing is typically more restrictive when it comes to headers, because of the location of the clutch fork.
Kurt