About your cable attachment...I would imagine that the connection to the firewall is simply a pass-through guide that does not actually anchor the cable other than keeping it stiffly in place...so...you might be able to fab a connector that simply guides it through and bolt it in place on the firewall...
Other than that suggestion...did the new cable not come with a plastic retainer with clips like the original? If so, it should just press into place and the clips would do the grabbing on their own...otherwise...maybe you can make one out of sheet metal that slips around the cable and then screws or bolts onto the firewall...