Will the x gears fit in a older non x case?

Yes, get the full upgrade kit.

How much do the X gears improve a 1998 B1?

Do you know how much stronger they are, are the floors stronger, etc?

That was the change. The floors are thicker.

I ran a 31 warlock cat with twin 850s and bravos and alway blew the floor off the forward gear on the rh drive, I drive it hard but , no hole shots, and watched the Re-entry stuff

When the XR drives came out they redesigned the helical gears as well. They made the foors thicker and designed them to beable to fit the older drives. Since then the broken floor problem is not an issue. The next weak link is the tower itself. The teeth on the gears are the same for the helical gears, just the floor is thicker.
Hope that helps..
Dick