Unless there is a lot more cracking, I wouldn't hesitate to add a couple steel plates for extra support and use the existing frame. Adding longer plates should spread out the load and reduce the amount of stress for any particular area. But, that is just my opinion and I'm not an expert on the matter. Lot's of options to ponder. You just need to decide what is best for your build.
Something else to consider, the frame (on a '59) should be stamped with the serial number on the driver side front frame horn. Just in case of title issues. One more reason to consider repairing the original frame.
John
