If the axle off the disco has the same width C bars as your 90 then its a straight swop over, some have 40mm thick others have 45mm thick, your calipers from the 90 should bolt right on to the disco axle, ive fitted loads of disco axles to 90s and 110s never had any issues, dont forget to move the steering damper of the disco axle as its behind the diff, yours should be fitted to the drag tube in front of the diff,