it largely depends on where you want to find yourself on a scale ranging from “approved ex-factory” to “will work … somehow”.
Using the 16" wheels as on sports versions of Jaguar XJS is the entry point. Arden used 8.5 x 17 wheels without modifications it seems.
Everything above is a question of trial and error. Unless you want to install steering locks and/or roll in fender lips you’d probably be limited.
BTW, going much further you’re not only compromising on ride, but also on longevity of bushes and all other suspension parts as loads will increase exponentially as tire ratio is reduced.
In terms of tire choice I’d probably go for a 17" - there’s more than enough at this size and everything beyond looks like another feature of “pimp my ride” or the Queen in high heels … do yo really want to see this?
75 XJ6L 4.2 auto (UK spec)