[Chris1529]

5 lug axle is easy conversion. You need axles with C-CLip eliminators, so order 5 lug axles. (5 x 4.5) ford bolt pattern. 94 and newer mustangs have 5 lugs, so get a set of rear drums for them if it still has drum brakes.

most racers hace converted to rear disks. order what you want, or re-drill the rotors.
same with the fronts. if you have stock brakes, Ford Racing makes a 5 lug conversion kit, or just order a lightweight aftermarket kit. your 88 has the larger spindles, so either option is viable.

you don't need a backseat. just carpet/finish the area.

gears are totally dependent on the engines capability, tire size, and desired RPM's. As I know nothing about the capabilities of that engine, I couldn't even venture a guess on this topic.

I don't have my 08 IHRA rule bok either, so I am not sure whether a turbo car can still run it SS. They did run SS/DX as you mentioned a couple years ago. If they do, the index is 9.35 for the quarter mile.
It would take a very healthy 140 cu inch car to run that quick let alone run very far under the index.

IMO, this is definately not a class to enter into the world of SS racing.