I'd get it reamed out to 26.6 or get a bit of wooden dowel, wrap a load emmery cloth round it, stick it down the set tube, attach the other end to an electric drill, whizz away. It'll clean the muck off the inside of your seat tube and you'll proabably find a greased 26.6 seat post will slide in effortlessly.
That's mechagouki. He has the two KHS Montana I've passed on to him. Otherwise I'd let you know the post size. Give him a PM, although he's been hard to reach lately.