Where does the shifter come up through the cab floor? Which transfer case do you have? Is there a spacer between the bellhousing and the T-18? Your drivetrain isn't factory for a 1981 J-10, so it's hard to comment without knowing what you have.
The 360 to 258 swap is a lot harder than you'd expect. Jeep accommodated the difference in the length of the two engines by using two different lengths of T-18. The input shafts and bell-to-transmission adapters are different lengths, by about 4 inches. To deal with this, you either have to move the transmission and transfer case around, or swap to the right transmission.
This has been discussed a lot in the past. If you search old posts, you'll find a lot to read.
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
ECO Green: '15 FCA Jeep Cherokee KL Trailhawk