Paul,
Your buddy must not know Advance-Design truck wiring. As twotone posted, the motor mounts have no affect on the normal/correct grounding.
As twotone posted, ground-cable/strap from battery to the frame behind the battery.
A ground strap/cable from the frame to the top of the clutch/flywheel housing was specified for 47-55.
Nonetheless, all ground points/connections should be clean metal (preferably treated with conductive, anti-oxidant grease); and, if your system is still 6v, the gauge of all battery cables/strats (including ground cables/straps) should be no thinner than 2 gauge (1 gauge, or down to 00 gauge (2/0 gauge) is preferred).
