I'm thinking about buying a vintage ('80s or '90s) frame for a 650b conversion. In my dreams I'm hoping to find a big frame (60+ cm) with long chain stays that was built for center pull or side pull brakes. I figure this means I want to go looking for old geometry charts and/or catalogs. I know about some sites, such as Vintage Trek and The Headbadge (although I can't find much geometry information there) – are there others?
For a 650B conversion, the length of the chainstay is mostly irrelevant, because a fat 650Bx42 tire has about the same outer diameter as a 700Cx25 racing tire -- so it's rare to have too short of chainstays for a 650b conversion. And most of the appeal of a 650b conversion is that you can get the handling and feel of a compact sporty bike with the softer ride of wide tires, so I wouldn't discount short chainstays from your search.
The most important thing to look at it whether the bike will physically fit the wide tires, and that comes down to the width between the chainstays and the width between the fork blades. These clearances are hardly ever given on geometry sheets, so you'll either have to take a look at the bike in person, or see photos showing how much clearance there is between the existing tires and the fork and chainstays. Remember that the widest point of a 650b tire is at a smaller radius than the widest point of a 700c tire, so a frame may have more clearance for 650b tires than it looks like.
The other thing to look at is how much reach your brakes will need. That's determined by the distance from the axle to the rear brake bridge in back, or from the axle to the hole in the fork crown in front. You want it long enough to clear the tire (+ fender if you're using fenders) but longer will just lead to spongy braking. If the bike you're looking at already uses 50+mm of reach to reach a 700c rim, it's longer than ideal.
The fastest and most reliable way to measure is just to try putting the wheels in the frame. If I were thinking of building a 650b conversion, I'd start by getting a 650b wheelset and tires and bring it with me while checking out craigslist frames.