I ended up replacing all of the hoses on this car, and that may have been a mistake. Today, at long last, I got the final touches done on putting this car back together, added coolant, and just my luck there is a leak at one (or both) of the heater core connections.
It was such a pain to raise this car high enough to connect hoses and tighten the hose clamps from below on the heater core. I also noticed that one of the connections on the heater core was slightly smaller than the other, however both hoses were 3/4" on the other end (one connects to the cylinder head, and one connects to the plastic crossover pipe).
I think the one which connects to the crossover pipe was actually a dual-diameter hose, meaning that the two ends are different sizes. I used a 3/4" hose with a pre-bent 90 on one end, and tightened the hose clamps using a nutdriver and two fingers to avoid over-torquing.
I guess at this point, I will need to either raise the car high enough to swap out the hose, or tow it to a shop and have a professional mechanic correct this. At least the engine started ok, and idles smoothly, though it does make a lot of valvetrain noise which was not there previously. I only ran the engine for 2-3 minutes for test purposes.