Do you already have the thumb and stiff arm David? Or is it gonna be a hydraulically operated thumb?
If you are fabricating the stiff arm mount I could give you some suggestions and weld on suggestions.
Crude drawing over coffee this morning.
Biggest suggestion is to weld to stick bottom plate as shown. And be sure Base plate bracket width is 1/2" inward on each side and inline with stick verticals to top and bottom plates of stick.
Also Weld top and bottom of base plate bracket as shown. Curl out both top and bottom stringers. And then flush grind to eliminate stress. DO NOT continuously weld top and bottom of base plate as it will create stick cracks of bottom plate. Three pin holes for stiff arm to create some manuverability.
Hope this helps a little on your design.
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