Well, Sheldon Brown does this sort of thing without problems. The spoke lengths won't be a big deal actually - you usually have 1-2mm of wiggle room in both directions on properly-chosen spoke lengths, and the differences from skipping a hole aren't going to be more than 0.5mm so that won't matter.
The real issue, I think, is whether the hub ends up being weaker because of it. If it's a relatively low-flange with the holes spaced very close to each other, the forces on the flange may be problematic. But if it's a higher-flange hub, I'd say no problem.
The other thing I'd say is that you can get good front hubs for cheap, so why bother. I got a Shimano LX front hub from Nashbar for $10 recently. Throw your 40-hole front hub on eBay - maybe it'll go for a high price since it's a rare/strange thing (but then again, maybe it's rareness and strangeness means you'll have few bidders) and just buy a 36-hole hub.
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"c" is not a unit that measures tire width