Basically, if you're running a single chainring up front, you can use some spacers to give you a little more clearance on a standard width bottom bracket. The OCC chopper BB has a very wide crank axle, but uses standard cups... you can buy the crank axle separately if you can make or find a wide 3-piece BB shell. You'll have to put some thought into rear hubs too, most won't clear a 3" or wider tire without an offset sprocket, etc... choppersus.com sells cheap singlespeed wide rear hubs, Sturmey makes an extra wide 3-speed rear hub with a disk brake.