91 octane to stop vapor lock?

It might be you haven't gone far enough, or that FUEL IS NOT the problem


You have a carb base heat isolator? The pump rear mounted? You been careful with line routing away from exhaust?

"What else" does the car have for smog equipment?

My memory fades, "might" have EGR, certainly has (had) carb snorkel heat, and various bullshit devices to screw with timing. Some carbs have temp compensation, and some of THEM are "trouble."