The length of the rear derailleur cage is the important bit, they can be short, medium or long.
Larger cassettes require longer chains which means the derailleur has to be longer to take up the slack when using the smallest cogs.
The Wiggle guide in the link explains it all...
As the others have said a spacer is required to avoid fouling.
Can't tell if the recessed nut fits into the fork, some have a smaller bolt hole.
If that's the case you need a nutted brake with a longer bolt.
Item isn't pre 1997 but most of the frames it will fit are:
I have a brand new black Thomson Elite 26.8 x 330 inline seatpost.
Unused and unfitted only taken out of bag for picture.
Change of plan means it's now the wrong size.
Cost £95 in December, asking £70 posted (UK).