98 Master Cylinder in 91

I know the swap of booster and MC has been covered millions of times, but what about just the 98 master onto the 91 booster?

Reason for asking: Got rear disk kit from ssbrakes.com for YJ application. They say the only difference in kits from YJ app. to XJ app. is they supply a MC from a 98 XJ cuz the stock old-style XJ one sucks. I'm assuming they're correct if they're doing it, but still looking for input from the XJ crowd if there's any real issues.
Thanks all....

What's better about the later model (95 up) m/c and booster is the booster. It's a double diaphram and the old one is a single diaphram. The reason you use the late model m/c is because the whole assembly, the m/c and booster, are the same length as the older m/c and booster. The newer booster is longer and the newer m/c is shorter, so they fit in the same space. The newer m/c may be better than the old one, but I don't see how, the benefit has been in changing out both.

Are you sure they're not sending both?