Radiator clamps won't be nearly strong enough, especially after you notch out the clearances. Shaft collars have substancial wall thickness and structural strength which you'll need to get things to work.

BTW- if you go the the re-weld route don't have it repaired using the same design, where the set screw pressure stresses the shell. Instead have the crack cut open cleanly with a gap if 2mm or so, and have substantial ears welded on either side (possibly 3 pairs rather than the normal 2) so it can be clamped closed. This is the time proven design that's been used on the front BB of tandems for eons. of course, as others have said, a decent weld repair involves a number of complexities so make sure you go to someone who knows what he's doing.