CVK's up to 35mm should fit your rails and spacers - as you've guessed from that I don't think there is another bike (apart from the 750) that has a 68-68-68 set-up.
So you could get ZZR600 CVKs (pretty sure they are 34 or 35mm) strip them down , strip you carbs down and use the cradles, rails and spacers to respace the ZZR to the 68 arrangement - then you need to sort out how to attach to airbox
FCR's you get 33mm from the SP range of the ZXR
ZXR750 also has the 68 spacings but these are now large body carbs - they might squeeze into the inlet rubbers but best bet would be to get SP ones to stand a chance again your facing what to do airbox end.
FCR's from the 750 can be used with trumpets so no airbox mod.
Ultimately you might over fuel or not get them set-up right but you wont know unless you try.
On my ZRX400 I have jumped from 30mm CVKs to 36mm with out problems - the 36s had dynojet needles in them ,ran with pods and standard jetting for a GPz900
I use 32mm TMR-MJNs so the CVKs got removed. I have fitted 38mm TMR-MJNs (large body) and these did squeeze into the inlet rubbers but the bike ran crap on them and I would of been jetting them from now until my pension
A rolling road would have helped but paying for that time would have outstripped the cost of my bike
MM!
