Correct post for a specific combination of frame and saddle.

Good day,
I am in the process of assembling parts for an Extracycle build. The
frame I will use is a Schwinn Clear Creek (please don't groan, it was
what I had available).
My question is this. The seatpost from this bike is stamped 26.6mm,
but actually measures 25.something with my (rough) pocket chump
calipers. It does not support the Brooks saddle I want to ride with.
(It's a Tioga, and appears to be made specifically for the saddle that
came with it)
Does anyone have any direct experience with this size seatpost, and
know if the clamp on a 26.6mm seatpost would ever be able to support a
Brooks B17?
Silver seatpost, to match the nitto stem and albatross bars a plus.
Thanks
Kelt.

Update: the rails on the saddle appear to be ±40mm c-c
again, measured with an "Executive Pocket Chum(p)" caliper