The steel welding line is what carries the voltage through the whip though, if you change it to teflon you would also have to create a new path for the voltage.
The spool gun works because its a short distance between the Alum wire spool and the welding point, the wire doesnt have to be pushed very far through the tube, which remains straight and doesnt allow the Alum wire to bend causing kinks and jams. Most whips are 6+ feet long though, and are able to bend and flex like a water hose. Pushing Alum wire from behind through 6 feet of bent and twisting teflon tubing is a recipe for disaster though, the wire is not rigid enough to hold straight for that distance without jamming. I hope for your sake it works though, put a video up on YouTube when you get the thing going. You are not the first person to try this though, and I think if it were possible welder manufacturers would offer an Aluminum ready whip to fit in place of the steel one.
Kinda makes you wonder why they went through all that work designing the spool gun when all they had to do is replace the guide tube on their existing steel whips LOL!
