Ok! After spending a few hours taking a booster out, putting the new one in, taking it out, putting it in again and yadda yadda till it was right I finally have a really damn good braking XJ.
Here’s the old and it’s shape
Heres the 84-90 compared to the 91 and up
If you ignore the protruding push rod, they’re pretty damn close to being the same. Except for one detail. The hole for the brake switch is too small. So I drilled it out. Got the size close to right, minus some welding and dremel work to make it close to the other booster. But I got it, brake lights work, brakes damn well. Even got the rears to lock up after a quick acceleration to 30mph.
All in all, it was worth the effort and no more failing booster and hopefully my idle will no longer be at 1500 when driving and starting. Which means I’m gonna have to adjust the ISA again at some point.