Honestly the frame on a back half car is usually not the best place to ground the battery. I prefer a floating ground system myself that brings all the grounds back to the battery basically. A voltage drop test will show where the issue is. I would NOT use just the motorplate mount as a ground. I would like to see a ground from hear to head then to the floating ground or at least a very solid place on the frame. Try an experiment and connect a cable from the battery to the starter to provide a ground and see If it changes. My guess is you need better grounds.