removing air tube

Try removing the bolts from the upper mounting bracket at the rear of the engine, and then loosen the clamp on the air tube by the cats. Then get a BIG pair of ViseGrip pliers (you really didn't hear me say that), clamp down hard on the tube about 1 1/2" from the clamp and pound away on the ViseGrips. It will come off if you keep at it.

In really stubborn cases, drill a 1/8" hole all the way thru the air tube 1 1/2" from the clamp, put a nail, pin punch or whatever in the hole, seat the ViseGrips up against the nail or pin punch and pound away on the ViseGrips. This is a better method, as you can do the same thing in reverse to put the old tube back on, with some clever means to cover or fill the hole (weld, braze, sheet metal screws, hose clamps and hi temp hose, etc.).

A replacement air tube cost me $44 at Advance Discount Auto Parts, plus the air check valve didn't come with it - another $17, so reuse when and where possible if your money is limited.