Getting some other things done with the instrument panel disassembled is a very good idea.
Duct hose tubing? Are the ends of the hoses just cut or do they have a snap-together at the end band clamp for a finished end? If they have the snaps, they were sold in dedicated lengths, IIRC. The bulk hose was in "a length". Might need to get the diameter of the hose and check with local auto supplies who might still have some in stock.
As to the ducts themselves, in many cases, the female end had some thin foam sealer that went inside them, which the male end indexed with, for a seal and better air flow output. Might be similar for the ducts which bolt to the hvac case? Browse the a/c duct seals at Home Depot or similar. Checking the operation of the actuators and doors can be done, too. Checking the condition of the rubber on the sealing areas of the flaps.
I suspect you'll find some traces/trails of coolant on the heater core, possibly. Might start looking for a new one. Might check the Online Imperial Club's website for sources, too.
I wish everybody well in that venture and hope it all ends well!
CBODY67