Whats the easiest way to transform 7 speed ultraglide hub (fh-7400) into 8 speed (fh-7402)?
(I d like to fit sti st-7400 dura ace shifters - I know I could go with campy ergo 8 + campy rear derailleur)
New cassette and chain but do I need new hub also or can I transform fh-7400 to fh-7402

Is axle length the same (one being 126mm and other 130mm) so I can only put mushroom spacer?
Or could I buy only hub axle with spacers (found it on ebay)?
What about freehub? (is it the same)
Is hub body the same?

The thing is I dont want to buy new hub and then respoke entire rear wheel if I dont have to.