I use the freddy fenders that Xootr sell for the Swift. They fit fine with 451 wheels and tectro long-reach calipers. I've only used narrow (Stelvio 28mm) tyres mind you but it looks like there's loads of clearance for fatter tyres. If you think about it, the reason we need calipers with such a long reach (when running 451 wheels) is because there is so much room between the rim and the brake/fender holes, compared to pretty much any type of bike out there using caliper brakes. So there's plenty of room for bigger tyres. 451x37 is not that big really anyway. The freddy fenders themselves are a very wide radius cross-section so will definately take much fatter tyres.

I have a spare pair of those tektro calipers if anyone needs a pair.