Well, it turns out that there would still occasionally be some fumes under wide open throttle. Since I KNEW the heater boxes were good, I took a second look, and found the original muffler was starting to separate. Where? The top seam on the left side of the muffler, and ironically the leak sprays directly at the spot where the "C" channel is missing, preventing the rubber seal from doing it's job. Since the muffler appears original, I wonder if that's what caused the channel to get eaten away in the first place... Exhaust fumes are moist and acidic 
So, no problem if it was real cold out and I drove moderately. If driven hard, the muffler heats up, the muffler seam opens up more, and I get more fumes shot at the only compromise in the engine seal, which gets sucked in by the fan. The original exhaust donuts were leaking too, and I suspect this is why the situation improved when the heater boxes were replaced and new gaskets and donuts were used.
I had to replace the muffler. A friend had an NOS stock muffler, but he also could get me a discount on new systems. I told him I wanted the stock muffler, but also ordered and installed a merged header and single quiet pack muffler from Bugpack. I will say that it sure quiets things down. The first thing the kids said when they rode in the car was "Boy, it's a lot quieter now". The quietpack is much quieter at idle and cruising (almost sounds like the engine dies when you coast to a light now), but it's louder under heavy throttle. The header did improve the low and mid range power after re-jetting the carb, but the very top end was not much different. Probably hitting the flow limit on my single carb, but I want to keep it stock looking as possible. It can out-muscle a modern Ford Focus in 1st, 2nd, and even 3rd gear, so I'm not hurting for power.
I will say the header reduces one problem I had, mostly because of the visual difference: Since it looked stone stock before, everybody, and I mean everybody, would avoid getting behind me at lights and try to swing to the next lane to pass. I would just drive off normally when the light changed and they would look at me funny. But now, they see the snake-like maze of black pipes and odd muffler, and don't bother trying so hard to get around us.
By the way, I did find out an un-pleasant but quick and effective way of testing the heater boxes. I cleaned off the end that goes into the muffler, covered the other end with the palm of my hand, and applied vacuum with my mouth. No, it was not tasty, and it's not the kind of thing you want your neighbors to see, but it did tell me that my previous heater boxes did NOT leak.
Paul
I remember the first time I replaced some old worn out heater boxes for some used (but in perfect shape) heater boxes I got at a junk yard...WOW what a difference. Like you described...I had all sorts of heat...and I had warm/hot air coming out of places I never felt before (front foot-well and the windshield defroster vents).