I just replaced my leaking Adwest steering box on the 420G, replacing the output seal did not fix the issue, so I inserted a previously rebuilt box.
Its a big job on a 420G, as the thru bolts support the engine mount, and to make matters worse, I find they changed the steering box and frame sometime between the '68 420G I personally pulled the box out of, and my mid '69 ( this is actually shown in IPL4 Parts supplement, with different part nums)
For whatever odd reason Jaguar reduced the thread size of the hole in the box from 7/16" UNC on the early box to 3/8" UNC on the later, and one of the other bolts has a spacer, replaced on later cars with a different body mount…so 2 of the 3 bolts are different
Luckily I was able to run a 3/8 bolt thru the 7/16 hole and put a nut on it to retain that one, and the others are open holes, so just adjust bolt length. One bolt is the same!
To fit the correct threaded steering box retaining bolt would require drilling out the engine and suspension mount from 3/8" to 7/16", that would be best possible way to overcome the issue
At least I appear to now have a leak free steering box, something I havent had for many years