I finally got some time to get back on this. I tied the hoses together with one of these:
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I put a short hose over one of the heater core fitting, with the other end into a bucket, then gently blew some compressed air into the other fitting. After getting as much of the coolant out as I could, I found the same puddle on the floor in the same spot. So heater core right?
I went ahead and removed the box (it actually was fairly simple, thanks to some tips I found on here like removing the glove box liner and getting that hidden hanger bolt off) and sure enough... it was perfectly dry and NOT leaking:
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So what I learned is, I'm not very smart. Oh wait, I already knew that. I learned that any coolant leaking near the header tubes, that runs onto them, will follow the tubes down to the collector and drip off there. So what I spilled when disconnecting the hoses appeared to be further back and fooled me.
I did take if for a drive with the hoses tied this afternoon and there is now no coolant leak whatsoever. So that's a big relief. I was starting to think it could have been a freeze plug at the back of the head, or maybe a head gasket. Just a dumb leak at one of the hoses I wasn't able to detect.
Thanks everyone for the advice, even though it turned out to be something I should have figured out myself. On to carb tuning.