I would (and did) just go with an 8-speed cassette. Before I moved to Europe, I cold set the rear on my 120 mm spaced '77 Centurion Pro Tour to 135 mm and put on a cheap Shimano touring-oriented wheel with a MTB hub on the rear and a dynamo wheel on the front. I don't regret it at all. The rim size was a non-issue for me, but my bike has brazed-on centerpulls and not the more frustrating cantilever brakes. I'm using a modern Acera RD mated to the original Suntour barcon shifters to pull the chain around and it works nicely.