First off, I ride a 1981 720 as my "regular" bike, and have since it first came out. You can roll up to 700X38 tires if you're willing to give up the front fender. In fact, I had a nice self-supported century ride in the coast range yesterday on 38's with about a third of my ride on gravel logging roads. So, if you decide against the 650B conversion, you can definitely go bigger than 32. In fact, if you have some brake posts put on the fork you can go to 38 with fenders since the brakes are what prevent the fender from clearing the tire.

I converted my wife's 1984 720 to 650B recently. Since her bike came with a uniquely poor cantilever set-up and was due for a paint job, I also converted her brakes over to R+E's Big Squeeze, which she loves. After using lousy brakes for thirty-plus years, she would not have been pleased to go with longer reach, and poorer stopping power, brakes. I put 650BX42 mm tires on her bike, but it would have easily taken the 48's I was considering. She is absolutely thrilled with the conversion. While I was making changes I widened the rear drop outs to take a standard cassette and changed her over to a nine-speed set up. She would tell you to go for the change over just for the improved ride qualities.

If you are willing to carry a spare tire, I don't see why you would hesitate to tour almost anywhere on 650B. Certainly in the US you can have tires delivered over-night, or in two days at the latest, for a small additional fee from an outfit like Compass (their tires are outstanding, by the way). Really, when was the last time you damaged a tire so badly you couldn't limp along for a hundred miles or so? If you're going to be putting in the kind of miles on a tour that require new tires along the way, you can always have them delivered to a post office down the road and pick them up when you get there. Having your bike set up in a way that pleases you and rides nicely is way more important than the once-in-a-lifetime back-up plan, imo.