650B's won't work on a pacer frame. A Disc Trucker or a Surly Troll can do a 650B. Soma has a new 650B bike frame that is pretty cool - The Grand Randonneur, designed by Boulder Bicycles.
Go to SomaFab and see what the frame sizes are. I would be concerned with toe overlap on the 42 pacer as it has to have a 700C wheel. Have you considered a trucker at all?