/6 sputters under load

Tune-up parts and technique suggestions in this thread. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download. My initial guess is that the carburetor (still) has problems. Idle mixture screw only 1/4-turn out isn't normal, even at 6k feet. Could also be a problem with your EGR system (is it intact? Removed? Partially disabled?). Is this an original high-altitude car with catalytic converter? Does it have an OSAC valve?