Thank you for all your help and suggestions along the way. I think I have the dual battery and the horn installed. Some challenges along the way, but nothing insurmountable - I hope. Next will be to go for a ride with the new modifications.
It is really tight getting hands in under the dash... wow.
Placed the horn above the left front wheel, I found a couple bolts that I attached it to that hold the body in place.
Only disappointment is the horn button that doesn't light up. It lit when I tested it, but when I actually attached the wires, no light. Puzzling.