I came along to serious audio in 1973, many years into the AR XB's lifetime.
The arm isn't anything like as bad as you'd think and a box style headshell is just right for low mass and reasonable rigidity too I think.
Look on the vinylnirvana/ar_models website for details on AR decks and their mods... You may be able to get a headshell from one of these people. They have pdf's of the instruction manuals too.
The best cartridges for one of these decks in the mid-seventies were the ADC XLM II which sounded lovely and the Shure M91ED, which was better to me for some reason than the 75ED which is supposedly mechanically identical.
Today, an Ortofon OM series cartridge would be worth looking at, along with the AT120E I think and possibly the Shure M97XE. As the only bias correction is composed of horizontal bearing friction coupled with careful cable dressing, you can't use too low a compliance artridge needing up to 2g playing weight. Something medium to high compliance tracking at a gramme and a quarter would be ideal.
Tear down these walls; Cut the ties that held me
Crying out at the top of my voice; Tell me now if you can hear me