Todd,
I would suggest checking your seat lug with an inexpensive digital caliper that can measure inside and outside dimensions in both metric and English units. I found one of these at Harbor Freight for about $15, and it has come in handy for lots of things.
I have a 77 Peugeot with Reynolds 531 French tubing, and the right number is 26.4 mm. However, my seat lug was out of round, and the bike shop had sanded down the post to make it fit. The post was being squeezed and scratched fore and aft, yet wasn't touching on the sides. I've heard that French bikes have a reputation for inconsistent seatpost sIzes, so perhaps the French were at times not too meticulous on the preparation and finishing of the seat lug. After all, the tubing itself was probably not the problem, right? Of course, if your post fits smoothly all around, like a piston, you don't need a caliper to tell you the seat lug is okay. But it is nice to quantify exact dimensions. Seat post sizing is pretty fine, the difference from one size to the next being about the thickness of a sheet of paper.
I made a set of cylindrical oak wedges to gently force my seat lug back into somewhat correct shape. I also applied the Dremel tool and some sandpaper on a 1-inch dowel to smooth up the inside of the lug. Without too much trouble I got the lug to accept the 26.4 mm post, maybe not perfectly, but pretty close, and without having to spend a lot of money (as in having the lug reamed).
Regards,
John Hurley
Austin, Texas, USA