Hi Andy
I had a similar question a few months ago when I refitted the engine in Connie. The original earth strap fits from the clutch bell housing/starter motor bolt to a bolt/stud on the body just below the radiator.
However, the advice I got from other members was that it doesn't matter too much where the earth strap connects from/to as long as the connections are clean to start with. Adding a second earth strap seams common practice, although I haven't needed to do this (yet).
Cheers
Gary