Or you could make some drop bolts a la Sheldon Brown. Could be more hassle than it's worth, to be honest - if the frame has close clearances, you'll still only be able to run a narrow tyre even with smaller 26" wheels. If you're really set on 26" wheels with fat tyres, get a different frame; if you're really set on using your track frame, use 700c/650b wheels.
Or you could make some drop bolts a la Sheldon Brown. Could be more hassle than it's worth, to be honest - if the frame has close clearances, you'll still only be able to run a narrow tyre even with smaller 26" wheels. If you're really set on 26" wheels with fat tyres, get a different frame; if you're really set on using your track frame, use 700c/650b wheels.