I had an interesting experience in refilling the SDL. With the back end sitting about 12 inches above normal (on a jack) and after filling the expansion tank to the very top and allowing the system to fill as much as it is capable of, I started it and put the heat on high. Let it idle and the temperature climbed up over 100C.

OK, so, how to get more coolant into it?

Took off the upper hose and poured coolant directly into the head, a bit at a time, until it was completely full. It took about another quart. Didn't seem like this would be the solution.

Started it again and the temp climbed up to its normal 88 or so.

Took it for a drive and it remained at 88-90.

Refilled the tank with another pint or so after it cooled off.

Apparently, the small amount of additional coolant that was added directly to the head was definitely necessary.