How about just below the heater motor and control valve? It looks to me like the battery tray could be attached to the firewall there. I don't think reinforcement would be needed, but you may need to modify the inner fender a bit for clearance. See the attached pic that I marked up. I tried to indicate the inner fender location with the dots and the area where you might need to modify in yellow.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
