I'm not sure if this is a stupid question or not but I am trying to replace the original throttle cable on a 1970 Plymouth Barracuda Gran Coupe 318.

My problem is the "original" (to the best of my knowledge) cable has this weird square plastic thing surrounding the throttle cable on the inside of the firewall, this plastic piece is not present on the replacement cable or mentioned in the mechanic's manual and is keeping me from taking out the old cable. It originally had a random rusty piece of metal running through it as represented by this rust colored line I drew in paint:

http://krogoth.org/cuda3.jpg

I figured once the metal was removed it would simply slide through the firewall and all would be well, but alas that's not the case. Now it looks like this but still will not push through the firewall in either direction;

http://krogoth.org/cuda22.jpg

This is what it looks like in the engine bay:
http://krogoth.org/cuda.jpg

I've spent an hour trying to break off the little wingtips, trying to force them through, pinch them together, pry them through from the engine bay side and everything short of just taking a hammer and busting the whole thing to pieces.