The aftermarket starts to get a little confusing with their different methods of mounting and providing clearance. It's interesting though.
(All in reference to my '69 here - sorry)...
Judging at least from Summit's site, it looks as though the
Addco is bent inward more (than
Helwig's at least) right before the ends, but mounts rather low on the arm, so this is necessary for clearance(?). The frame mounts look higher on the Addco and so doesn't require the kink in the middle to clear the k-member-to-rad-support bracket.