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I would say yes if you use the XJ6 uprights on the front as the calliper mounting points are different.
Two problems with this.
First problem is the callipers being bigger bind on the inside of a standard 15 inch steel wheel and I had to grind the edge of the calliper away so the wheel would not rub.
Second problem is the uprights although they fit the subframe, they are a slightly different set up meaning the distance between the upper and lower ball joints is longer so the spindle is set lower on the upright making the front end sit higher. When I first had my S Type a friend of mine offered me the front end from an XJ6 with the callipers which I fitted and ran for a while but could not get the front to sit down so the tyres would fill the arches. Always had a big gap at the top of the tyre and the bottom of the front arch. I then changed from steel wheels to wires and the wire hubs and spindle would not fit on the XJ6 uprights so I changed back to the original uprights and brake callipers.
Below is a photo of the XJ6 upright next the the S Type upright showing the difference in distance between the upper and lower ball joint mounts.
As Cass says ^. In addition, the XJ uprights have the ball joints a little closer together. To maintain good geometry, it's a good idea to turn the top wishbone (including it's mount) upside down. It's possible to lower the car by putting spacers between the spring pan and the bottom wishbone. However, why bother? There are several brake upgrade kits that can be fitted to the original uprights, not to fitting mention the 420 suspension. The only reason I can think of the XJ swap is that it might possibly make an XJ rack work better.
The Coopercraft website has disappeared. SNG sell a kit, presumably made by another company. Zeus, Fosseway, BCC, Wildwood are still around.
I'd thought of using XJ uprights as they might have given better steering geometry with the rack with a brake upgrade thrown in. The cost of all the secondhand parts plus some reconditioning was looking a bit high. As it happened, I found a rebuilt 420 front subframe complete with Marles steering box, suspension and later brakes. This saves me time and money - both of which are required in large amounts to tackle the corroded body of my car.
