I used a switch plate from the smaller light when I restored the 5SW which had an 8" light...I can't remember whether the curvature was absolutely identical..it might have been. I certainly didn't struggle to fit it...Ian
You are right Job. The curvature of the ammeter / switch holder of an original 6.5 inch headlight is different, compared to an original 8 inch headlight. You can easily adapt the ammeter holder of an 6.5 inch headlight to an 8 inch. Put the ammeter holder on a bench (before you paint it) and slowly put some pressure on the holder from above with one hand. The curvature will change, then check on your headlight if the panel fits, otherwise repeat the practice until it fits.