Following under the eaves of the house it is about 70 feet to the box. This seems a long way for a good ground connection. A shortcut through the attic is possible but the room closest to the box has a vaulted ceiling with no attic above it.
There is a much closer location that was where the house's original box was. Any likelihood that the defunct box (now full of nest of wires) has a proper connection to ground?
Another option might be a cold water spigot that is quite close to the antenna.
EDIT
I can change to location of the antenna to the analogous spot on the opposite side of the house. It will be a 40 ft straight shot to the spot (below) where the cable box ground is.
Adding two pic of connections to water pipe.
Primary connection
nearby second connection of gnd for cable box
What is in the spot of the original box
The new box environment
Best Answer
I would mount the antenna on the same side of the house as the current box, and run straight-shot to the cold water bond point
Given that you can't rely on the integrity of metal plumbing to bond things together, your best bet is to place the antenna so that you can run the antenna ground wire as a straight shot to the existing water pipe bond point. This will give you a decent place to tie into the grounding electrode system at, provided you clean up the paint and such so that a good connection can be made with a split-bolt or other listed connector; with a bit of work, you could even retrofit an Intersystem Bonding Termination device there.