I know this is old now but for info when i converted my xr4x4 to 4x4 cosworth hubs i ended up having to change virtually everything to get it to work .
I used cosworth front hubs , cosworth front drive shafts , cosworth roll bar , cosworth steering rack , cosworth front calipers .
I tried with just the hubs first but couldn,t get them to bolt up with xr4x4 shafts . once i had the shorter shafts and hubs on i found that the wheels hit the steering arms and roll bar on full lock so had to change to cosworth steering rack ( less turning circle to take 7j wheels) .
Once all changed it obviusly worked a dream
regards
Dave