I'm going through a similar, though not identical, process.
It's worth repeating that steel frames, especially lugged ones, are very amenable to adaption. If you look at Ellis Briggs and Bob Jackson's websites (just as examples, they're near me) they can do practically anything you want to a steel frame. There's a cost, of course, but things like mountings for V brakes and various other braze-ons aren't a fortune. And yes, they can spread the rear dropout to take a wider hub. Some of the Dawes Galaxies had a 135 OLN I think, but caveat emptor. Caveat Emptor is latin for take a tape measure with you.
There are a few 'gotchas':
Older French bikes are a bit non standard (and watch out for the 'quill of death' on Peugeots). Seat posts are different too.
Raleighs also. Especially the 597mm wheels they used on some bikes. But the good thing about Raleighs is there are about 4 billion old ones. So you can probably get what you want from scrap ones. And people still makes parts for them, e.g. Phil Woods' bottom bracket. Sheldon Brown has some good articles about old Raleighs. His opinions seems to be they were so well made that things didn't wear out anyway.
There's probably an optimum age to get a good traditional steel frame for a project like this. I'm guessing 1990s would be a good age. You can probably get a Reynold lugged steel frame, but fittings would be fairly standard, and the price would be not bad. Lugged frames seem to drop the price a bit, cause they look really old, like your dad's bike.