Hi folks, I picked up '74 Sekine a few weeks back and have been planning on swapping out the seat tube. Current one has a separate clamp, and just want something something easier to adjust and a bit slicker looking. But I'm having a bit of trouble with sizing.

The existing tube is marked as a 25.4, and measuring it this morning with a micrometer puts it pretty much at exactly that. Digging around earlier posts on this site, it looks like the tube should be closer to 26.4 - 26.8 in sizing. Looking at the top rear of the tube, at the gap at the back it looks like it's been "squeezed" together to accommodate the smaller post. (Can't post photos unfortunately!)

I guess my question is what the best course of action is here. I was planning on trying to stretch the opening out so the gap at the back of the tube is roughly parallel, and go to the LBS to see what the tube size really is. But not sure if having such a smaller post in there would mean about the integrity of the tube. Bolts and fittings are quite stiff so want to know what I'm doing before trying to wrench things apart. With regards to replacement posts, ~26.6 seems like an easier and cheaper size to come by - and would be "correct", not that that matters too much.

Thanks for any advice!