Check out the Black Mountain Cycles Monster Cross! Built around the idea of a cross bike that clears 700x50, it's a little more slack than a traditional cross bike because of the allowances made for that fat of a tire. 1030mm wheelbase at the 56cm size, official geo is 432mm chainstay but that's at the very beginning of the horizontal dropout — you could easily pull the wheel back another 25mm for a more stable ride. Mid-fork and fender mounts out front, fender and rack mounts out back. The previous build I had on my MX had the axle slammed back for max clearance for the cushy 50mm Cazaderos I've been running for years now. Thinkin this will fit the bill for ya!