Is there a clamp somewhere between the firewall and transmission clamp?
Yes!!
The freewheeling cable goes thru the firewall and then it runs down against the inside "leg" of the cowl. Midway down the "leg" of the cowl there is a clamp that attaches the freewheeling cable to the cowl. From there the cable loops down and it comes up to the side of the transmission. The cable is then attached to the clamp on the side of the transmission. From there it continues up and connects to the freewheeling lever on the transmission.
Without the clamp on the inside leg of the cowl to hold the cable in place, the cable will be in the way of other wires and etc.
