It is probably a 26.0 seat post that you need. A lot of older bikes have inner seat tube that are not perfectly round(off by fractions of a mm), that combined with a little corrosion and seatpost clamp tightening over the years can make putting a post in a bit difficult. You can try sanding the inside of the tube a little bit to clean it up(just not too much), I'd turn the frame upside down to do this since old BB are not sealed and the grit will get into the BB. Make sure you clean out all the grit you can afterwards. After that spray some wd40 into all the tubes you can reach and let it sit for a day before putting the seatpost in. The WD40 will help prevent any further corrosion inside the frame.