i did the dual diaphragm upgrade in my 90. i got the booster from a 99 xj for 30 bux from wreckers.
the brake pedal push rod is a little longer, can swap out with your original, or just leave it. it's not that bad and easy to get used to. i just left the longer one in on mine.

but you do have to ream out the bolt hole on the end and also grind the end flat to fit your brake light switch.

also had to divorce the brake differential (proportioning valve assembly) and relocate it to the stock position and have a new longer brake line made up and bend it to the other side of the master. the other brake line can simply be bent to fit.

that's it. easy job and huge braking difference. i can now lock up my 36's.