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I think I'd add a cleat on the outside.
Steve

TexasKZ wrote: I know that frame notching is common, but I wonder how much strength is lost in the process?

It does look a bit drastic but once it's boxed back in with the associated bracing (not yet done) that goes along with this sort of build it's not going anywhere.
You'll notice that the s/a pivot has been line bored for a larger spindle with zero play unlike the originals 0.4mm slop and the support area for the swingarm cups has been increased too.
Once it's all bolted up it will be as strong or stronger than before..