Interesting project. I keep thinking of trying a 650B conversion but haven't gotten around to it. Seems like it would be more cost effective to convert an old school rigid MTB frame to a sorta-gravel-ish drop bar bike.
My only concern with squeezing fatties into bikes built for skinny tires is the handling -- besides the risk of tire rub on the frames. Tire rub on chainstays of steel frames is no big deal -- it's just like riding with dragging brakes. But I have a couple of pre-owned carbon fiber road frames on which previous owners apparently tried to squeeze in 700x28 or wider tires without regard to alignment, dishing or ... pretty much any tech considerations at all. I'm not sure how they managed to ride the bike with the rear tire rubbing so obviously against the chain stays. Fortunately they only rubbed the paint, no apparently structural damage. But you'd think they'd have noticed the sensation of riding with dragging tires in pursuit of a "nicer" ride.
I *can* squeeze Continental SpeedRides into my Univega Via Carisma, but only because the nominally 700x42 tires actually measure closer to 700x38. Anything wider hinders the swing of the front derailleur.
The problem is all my good double wall wheels are 622x14 or so, intended for racy bikes and skinny tires. Sure, I can fit the fatter tires on the skinny wheels. But the handling is sketchy at speed on curves, especially on rippled and rough pavement. Too much tire overhanging those skinny rims.
I need to get some 622x19 or wider rims and try again. Meanwhile I'm maxed out at 700x32 Conti Contact Speed tires on the Univega, which ride about like lead filled garden hose. Durable, puncture resistant, decent grip, but harsh at full pressure and sluggish at reduced pressure. Not fun.