Yes less talk of morning wood plz!
Okay so here's an update; I got hold of some wheels for some test fitting; a pair of 520s from an Airnimal and a 650c for the rear; I think this may work well - the rear might be better with teeny-bit-smaller 26" mountain size as I think I can source matching Panaracer tyres and Velocity rims for the two disparate sizes that way. Anyone that follows my bike posts must know that something like that really matters to me!
So here's some pix. There's a pair of flipped north-road bars for spice (though I don't intend to use that stem) and a couple of test bridges made by dremelling a Berol pen tube to fit.
What's next? Hmm... Sourcing parts is all a bit Catch 22; Can't decide on BB solution and width till I work out what the optimum chainlines are; Can't do that without the right rear hub, etc etc... Need to get some brake calipers to sure up the bridge placement and then get the lot off to someplace that will braze them on along with some cage bosses, derailleur boss(es?) and perhaps where they can chrome the dropouts and forks. Lots and lots still to do then but I'm not in a rush; Looking forward to the arrival of a pair of Terry forks most generously donated by the true 520 originator - Guy Retreau. Once I have those babies I can get the headset, and front end a bit more together which will be interesting. I may well be stealing Phatatude's stem solution too, though my hope is to get the stem riser threaded so I can still fit a period quill stem rather than an a-head which I'm not sure would fit the style of this build. (ie to pass it off as an original item).
Side profile. Shaping up nice with pursuit wheel mismatch and flipped northroad bars.
Other forks in shot are 24" mountain-bike forks. These are basically too big for the 520 size - would need überlong reach calipers and I want to fit little road ones.
The forks test-fitted here are actually 20" Pashley trike forks (for 500a wheels). They are a few mm short for the job. At a pinch they'd work if you did a bit of grinding at the crown (Danger! Danger!) but I'm more than sure that the 24" Terry forks will be perfect so no need to put my life at risk with auto-separating forkage.
Test-fitting prototype brake/stay bridges, made from pen tubes ground with the trusty Dremel.