actually, he said he doesn't have wheels.
You want to stick to 120-126mm rear hub. Whether you use a freewheel or switch to a freehub/cassette depends on what you can find/afford.

I strongly recommend 700c over the original 27-inch - your stock brakes will have the adjustment to cover it easily.
(and if you don't have brakes, buy long-reach)
This would give you a world of tire options.

You can easily use an Ultra6 freewheel with a 120mm hub. With a 126mm, you can go up to 7 in the rear.
It would be a good idea to cold-set the rear triangles if you do go to 126mm.

Vintage SunTour would be the best choice for rear derailleur with friction shifting.

If you have a big budget for wheels, you can build a modern freehub/cassette wheelset
Compass offers these hubs and cassettes https://www.compasscycle.com/product...mponents/hubs/
With a correct new derailleur hanger, you could go to a modern Shimano-compatible RD with index shifting if you wanted.

You should also check wheel options at Velo Orange http://store.velo-orange.com/index.p...eels/700c.html

Across the board, front hubs are 100mm

If you go with a conventional narrow freewheel hub, I still have a VG Ultra6 freewheel around here, 13-27t