I have a bike which I *think* is also A Bianchi Rekord 748 (i tried to post an image but this board only allows images after a threshold of 10 posts.....)
I have the original seat post but it has no markings so I do not have a reliable measurement for the seat post width.
I ordered a 26.8mm diameter seat post but on testing it I can confirm that is too wide.
I also tested a 26.4mm diameter post which seemed to fit easily.
I have ordered a 26.6mm and I hope that hits the sweet spot.