First of all, that bike is awesome. second of all, best thing to figure out, in terms of sizing, is to stand over the bike and see how much nuzzy clearance you got. ppl always whimper about how sdtandover isn't most important, but guess what? It is probably the most important. The issue comes in when ppl act like it's the only important aspect of fit. Next, you'll want to adjust your seat height and possibly swap out your stem for a longer or shorter one. finding quill stems in various sizes is still possible, but they're not as plentiful as they were in years past.
You may be able to swing your pads enough to fit the 650b rims, but possibly not. Either way, you won't likely be able to cram neomoto tires in there. I'd try to rock the 26" with that frame. I mean, it was $6, right? nothing ventured, nothing gained...