Best place for ground under dash?

You asked for "the best place" which leads to opinions and other BS.
If the added weight/cost of wire doesn't matter, all the way to the battery is "the best place"
The factory grounded the heater case with a screw behind the right kick panel. To copy that on the left is a very good place. Not that anywhere in the dash wouldn't work. It is attached to weld up chassis with 7 bolts, U nuts over paint at 5 of those.