To find the engine ground strap your going to have to look for it. It is usually bare (no insulation) heave duty flat battery cable. It may be from the frame to bolts that attach the starter motor, transmission, or the firewall to the RH engine head stud. Since your car is not original but has several modifications, it is hard to tell where it may be. If you do not find it I would suggest you look in the Early Ford V8 Club restoration book for it's proper location and place one there. Page 5-15 shows the battery + being connected to the firewall on the RH side of the battery, and from what appears the bolts that mount the solenoid to the firewall to an engine head stud.
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