If you are still thinking of building up a 40-spoke wheel, I just was digging around in my parts box today and found an extra NOS (laced up but never ridden) Shimano HF-05 40-spoke hub in 145mm. It looks like it can easily be respaced to 135mm by simply removing 10mm worth of spacers on the non-drive side (you'd have to cut the axle down, too, which is easy). This is a tandem hub, but it is NOT threaded for a drum brake on the left side as most are, so it looks like a regular MTB hub. Drop me a note if you are interested.