X-6: Which upgrade to choose?

[General Accident]

I now have a mint 3-gear X-6 (thanks Jim) and want to convert to a 4-gear version. Is there much work in altering the chassis if I go for the 2nd stage upgrade, or is it easier to shell out a few extra quid for the 3rd level one that comes with a new chassis? I suppose that way I could keep my 3-speed chassis as a spare.

Also, is there a benefit to having 2.35 inch hingepins? What bits would I need to convert to them if they are a "must have"?