Having successfully filed the front dropouts last night (thanks AndyG and Biggsy!), I attempted to fit the new wheel. It's 26 x 1 3/8" (597) but has such a wide rim that it can't be fitted in the current brakes - well, you can have one brake block but not both... Even using the slimmer blocks from another bike doesn't work.
I seem to recall someone looking at the rim and saying "that's a cycle speedway rim". IIRC, that's a brakeless sport in which wide, tough rims would be a plus. Can anyone put me right?
So i have a wheel that is unusable for normal road purposes. OK, I run the bike fixed but I'm not going down the brakeless route. Something is telling me to give up on wheel sizes that are in keeping with the 1950 frameset and go for 700c wheels
26 x 1 3/8" is actually 590mm. 26 x 1 1/4" is 597mm.
I've just replaced 597mm rims on a bike with 590mm rims as tyres and rims are readily available. I've used very nice Sunn CR18 rims in 590mm. I can measure rim width later today if you like? They fit with Alhonga deep drop dual pivots.
If you really want a 597 wheel I know that Walton Street Cycles in Oxford has some, but they're steel rims.
If you go for 700c wheels on such a bike (I'm guessing it's an old "sports touring" type bike - mine is) then you'll raise the bottom bracket and possibly affect handling a lot.
I'd go for a 590mm rim : it's what Pashley fit to all their 3 speeds and you can get Schwalbe Marathons in that size. However these are very wide (37mm) and rubbed on my chainstays. The Schwalbe Delta Cruiser, while nominally the same size is 34mm wide and fit in my chainstays fine.
Hope that's not too confusing!