[84LX89GT]

yeah it's most likely a 7.5" rear, the current V6 mustang uses that rear axle and it was used in 5.0L applications up till '85 i believe. You can find some 7.5" rears from rangers with 3.73 and 4.11 gears as well for a good price basically new (if you wanted to dissassemble and swap over the ring gear) and also alot of the 5.0L equipped mid 80's mustangs that had the 7.5" rear end had traction-lok limited slip units. My '84 5.0L that i had, had traction-lok with 3.08 gears.
I think there are some oddball early 80's 6 and 4 cylinders that came with 6.75" rear ends, which are absolutely useless for anything above stock performance. You also had the early 80's pre-overdrive transmission rear gear ratios like 2.27:1 and other crazy stuff like that.
One way to find out what axle code you have is to look on the door tag under axle and see what letters/numbers you have and to look in a referance book like chiltons, haynes etc.
You can also swap in an 8.8" from '86 and newer 5.0L's and it's a direct bolt in, although you may have to get new rear brakes lines for the larger rear drum brakes. If you feel like spending some money you can get one out of a new mustang, or '87 to '88 thunderbird turbo coupe with rear disc brakes and swap over the master cylinder from a thunderbird turbo coupe as well as eliminate the metering block that is meant to apply the drums first.
The 8.8" rear axle is meant for more severe abuse and will handle an incredible amount of torque before they fail and are real easy to upgrade and make stronger with a re-inforced cover and welded axle tubes.
If i were you though, i'd probably go the cheap route and get used gears (it's easy to see if they're worn or not, pattern easy to see on ring gear) as well as maybe a new or used limited slip unit depending on if you get enough power to spin the tires.
I hope this was helpful, if you have any other questions feel free to ask.