If you're replacing the two male threaded pipes anyway, take a Dremel cutoff wheel and cut the tubes off to free the remaining smog tube assembly intact. Let your PB Blaster soak overnight from both sides of the fitting (shoot it back into the extension tube underneath). Then use a good quality 6 point socket to back the nuts out - carefully. Too much torque and you'll snap them off, forcing you to drill out the remains of the nut. I have used the successively-larger-drill-diameters routine until you can pick the thread remnants out - a painstaking process, but worth doing to save the integrity of the threaded port. I only use the tap to chase the threads after clearing, not to drive the remnants out. Just take your time, don't rush it, and think about what you're doing before getting frustrated with it. Derned things are too expensive to fix or replace - that's why you're doing this, right ?
Regards,
Steve