Out of curiosity, I looked around and did find some BMX brakes that were long enough. But the Tektro R556's have a 73 mm reach, so they'd be way too short for a 27" to 650b conversion, at least on my bike.

Anyway, I get it... You like 650b, and that's OK! Even if it's not necessary in this case for wide tires or fenders (and who knows if the original poster has any interest in either). Even if it might require a custom-built wheel for a 5-speed freewheel or sub-130 mm dropouts. Even if it requires replacing brake calipers with BMX models. Even if all these accommodations for 650b wheels incur more cost than the value of the bike. You're certainly entitled to your particular likes. Like I said earlier, we'll just have to agree to disagree.