as far as I know the older x3 accents used a different strut than the newer x3 accents. I'm not sure of the crossover point but I know there is a difference. The older x3 had a smaller diameter strut vs the newer which is the same diameter as the tiburon and elantra.
I have the beta brakes on the front of my x3, I didn't do the swap myself because when I bought the car it was already done.
To be able to use the beta calipers and rotors you have to use the spindles since the caliper mounting holes won't be the same since your using bigger rotors and calipers. The beta spindles should all be the same the only difference is the older ones used captive rotors the newer ones didn't, the spindle itself was the same though the bearing and hubs are different obviously.
if I can find out more info from gimp i'll let you know. The only thing I can think of is you have the older struts on your accent and this is causing your fitment issues at the strut mounting part of the spindle.
2000 gs- beat up dd(rear sway bar,kore exhaust, hve duct,beta front brakes, 14"elantra 5 spokes, hankook rs2s, cracked kore carbon hood, 1 kdm headlight)
1999 L- fsp terror (too many mods to list)