I just picked up a beautiful chrome 84 Mt Hood. The seatpost is not original, and I can tell it's too small. Someone has tightened the clamp in and made it kind-of work, but I want to get the correct post for it. Anyone know for sure? I don't want to measure it or start trying different posts, because I can tell the clamping area is deformed and the correct size post will go in with difficulty.

The post that it came with is 25.4mm. I'm guessing a 25.6 or 25.8 will work, but those are such unusual sizes I thought I'd ask first.