By "25.4" and "26.4" do you mean the size of the post or the size of the hole?

The four (50cm, 2x58cm, 60cm) I'm working on right now are 25.8, 25.8, 26.0, and 25.8 (all hole size).

They're all bullet-pointed seat stays, chrome front and back, look the same as yours (well no they don't, but they look to have the same frame). Three of them are 4 digit serial, all close; one is a three-digit.

If you haven't already, make sure the slot sides are parallel and measure again when they are.

And if you are doing a complete make-over, weigh the bare frames and forks; I'd like to compare.