If it's a T18, then it didn't come from an 84, but would have been an option in 76. You'll need to figure out what it came from.
Do you still have the Dana 20 transfer case or a D300? If you have a D300, its most likely a Ford transmission and a Ford bellhousing will bolt to it.
If there is an adapter between the transmission it's probably from a 72-75 CJ. Reverse right and up beside 3rd and 4:1 1st gear.
No bell adapter and a D20 could be a 76-79 CJ or a Ford. If its a 4:1 1st gear its from a 76-77 CJ. A 6.32 1st gear could be 77-79 CJ or Ford. Any 77 and older T18 will have reverse right and up by 3rd gear. Any 79 and newer will be right and down by 4th gear. 78 seems to have been a crossover year.
Once you figure out exactly what you have, you'll be able to figure out what it will take to adapt to a different engine. You could swap in an AMC 360 without changing anything, no matter which version you have.
77 CJ-7- AMC 360, Edelbrock cam, MC2100, Fenderwells, CJ T18/D20, Chevy D44/EB 9" rear, Trutrac f/r, 4.11, SOA, 1.25 lift shackles, 35x12.50 BFG KM2s, Shackle Reversal