V8's often crack in the valley, above the lifters. Usually not worth fixing, but a 426 is worth the nickel. A hemi head has a large flat top surface, flat places are the first to bust. The sides of the block usually crack on the outside and don't cause internal leaks.
Are you sure it froze up? A leaking head gasket can also cause oil in the rad. Does it get coolant in the oil? If not, it's probably an oil passage that's cracked, that can be a bitch to find, let alone fix.