I think your assumption about grounding between the pieces is correct. You could test this with a multimeter if you really cared to, but grinding clean both sides and then clamping to them should create the necessary continuity, since one side of the clamp will be touching each of the pieces. Here's an alternative, that will save you from grinding off very much powdercoat from the loader frame: grind clean the back surface of the frame, then clamp as depicted in the drawing. Grind clean the paint/powdercoat from the joint, then strike an arc on the frame side of the joint and carry the puddle over to the loader frame, creating a tack. Once the pieces are tacked together, there will be ample continuity.