Heat with an oxy-acetylene torch (propane will not cut it -- have a shop do it if necessary) until it is cherry red, and melt some paraffin into the interface between the fitting and the manifold.

You can also cut the tube, and just plan on replacing the tube -- the 'magic' is in the fittings, not the tube, per se. You can replace the tube itself with large-diameter steel tubing (summit should have fuel line, or the like) or even copper tubing (less vibration-resistant).

Once you have the tube cut, you can use a tight-fitting socket (hammer a 6-sided socket on, if you have already buggered up the fitting) and an impact gun.

Randii