there are crowns wide enough for 42mm tires, but supply can be spotty. Pacenti offers a couple of crowns, and there are Long Shen crowns that will clear 42mm. I assume that most crowns used on CX bikes will clear a 42.
As far as BB shells, I am not sure which angle he is talking about, as far as clearance between the stays, there are various ways of doing it.
I am just working on my first 650b frame, I am not using tires that big though
Tubing sizes and styles change things a lot. Are you willing to use s-bend chainstays? Are you set on having a lugged frame? Standard, oversize, or double oversize tubes?