[Tony99]

hello Huey, 51 Mercs don't have a specific earth strap from engine to firewall like the earlier cars. There is an earth strap from battery to engine, then from solenoid a heavy gauge wire like that link you have straight to the starter motor.
Like Rotor wrench mentions there is a lot of bonding already and my straps are correct size & in good shape. I was always a bit unsure as to how the engine bonded to the frame & firewall as it sits on rubber bushings but I guess there are the bolts of the water pumps to the frame which I assume then earths the firewall.
Seems like these cars have "switches" that earth to the metal rather than a seperate earth on the switch like the starter button & dome light switch & the courtesy light switches that press in as you close the door. I replaced my door switches with nos ones & then had an issue where the dash lights would stay on with door closed. Happened after a few months. I discovered that one of the switches wasn't making good contact on the door pillar as when I wiggled it lights would turn off & on. hence I thought an additional earth strap to firewall may fix a possible degraded earth?. I removed that door jamb switch since needless to say hard to remove without breaking it which I did so now will try with another switch. I did notice the replacement door pillar switches that have a plastic body have a 2 wire connector so one can be earthed to back of dash, rather than rely on the body of the switch making contact with the metal as an earth. As others have said proper ground is important.