Seat post concerns

I'd be grateful if you could give me a little advice; I'm having difficulty finding a suitable seatpost for Raleigh which has a 753 frame and was built by the Special Products Division .

I've tried a 27.2 seatpost, but I feel that I'd really have to force it in for it to fit. I'm reluctant to do so for fear of damaging the frame. The top of the seat tube is slightly 'ovalised' - no doubt due to previous tightening of the seatpost bolt over the years. (I haven't tried to force the bolt flanges apart for fear of damaging the frame).

I've taken the frame to a bike shop and they've found that a 27.0 goes in okay, but it might not be correct, due to the 'ovalised' effect at the top of the seatpost. ie. A 27.0 post might be okay at the top of the seat tube, but could be too slack further down.

Should I fit a 27.0 seatpost or try a little more force with the 27.2 seatpost ?

I'm told that 753 tubing is quite fragile stuff and I'm worried about damaging it.

Can anyone give me any advice please ?

Thank you !!!

You need an adjustable reaming tool that goes up to 27.2mm,any well equipped LBS workshop will have one of these,this will clean up the top of the seat tube very accurately.Don't be tempted to fit a 27.0 post !

ive heard of some 753 frames using 26.8. I think I read it on here somewhere where there was a discussion on this topic. See if you can do a search.