Going from 630 to ... 559? So the bottom bracket will be ~70/2mm or 3.5 cm lower to the ground. Hope my math is right. However with big fatso tires, you may make up some or all of the difference.

Front brake is another story, if you plan to use one. If you're planning to use a drum hub, no problem. If the plan is calipers, you'll need a brake with 3.5 cm, maybe less, more reach than the brakes you have now. Measure the distance from the front brake post to the center of your current brake pad (I think this is the right method). Add half the difference between the bead seat diameters of the new and old rims, and that's the total reach you'll need in a new brake. Here are some options, conveniently listed by reach. http://sheldonbrown.com/harris/brake-calipers.html

BTW, are you sure this is the bike for the job?