You'll run the risk of damaging the frame, because you'll need to clamp the post up too tight in order to stop it slipping.
You could make a shim out of a drinks can, but personally I'd sell the KORE post and look out for one in 27.0.
Thank you ....sort of think i knew the answer just didn't want to sell the Kore...
Ah well....any sugestions for a period GT suitable alternative...?
So far I have a CK headset, a set of Manitou MRD Mars forks, 221s on LX ( probably uprade these )
Kore or Azonic stem, ClubRoost or Azonic risers, or maybe some RF straight bars.
WTB SST-K, Avid SLs or Dia-compe VC767.
XT m737/9 groupset...though may change this to m750.
Tyres, grips, grips etc will most likely be modern stuff.