Rears swap

[69moredoor]

Hello all. Just picked up a 79 d100. It’s a 318/auto / 8.25…has 2:90’s In the rear. I’ve been researching options for a steeper rear ratio
I’d like 3:55’s and I know xj’s are plentiful with 8.25 with 3:55’s and same bolt pattern. I believe the widths are close. Anyone ever do the swap (I figure I’d need new spring perches)
Chances of finding an 8.25 from a similar truck with more than 3:21’s?

Thanks

 

[65 sporty]

A 9 1/4 should swap in there and they have lower ratios

 

[69moredoor]

Thanks. I’ve been looking and am confused. What’s the years with the bolt pattern changes

 

[7mopar]

Should be the same till 85.