Hi Ted, having worked in that space restricted area before, I think I would be tempted to bite the bullet and get the OE version, at least you know it's going to fit just right. Before you re-connect the multi plug behind it, take a little time with some small jewellers files and clean the terminals inside both parts of the connector. This was the cause of many blown main fuses for me a couple of years ago due to the pretty shade of green they achieve over the years! I wound insulating tape round it afterwards to try and keep out the worst of the elements. DON'T connect the earth lead before you have done this!! Your earth lead looks about right, and yes, connect it to the engine mount. Knowing what a problem bad earthing can be, I think it's worth sacrificing a little bit of paint to achieve this.
Just as an aside, I am in the process of tidying up and checking all the parts of a 1966 CL160 USA model at present. I hate to say this, but the build quality and strength of that era far outweighs even our 1970's possessions. The only piece of plastic I have found so far is the rear light lens. Plus of course, it ain't so bleedin' heavy. Stay Safe. Mike.
