Chuck,
Unless you happen to have a Dayco to industry standard cross reference chart, for the Dayco number to be useful, you have to find someone who sells Dayco and advertises them as Dayco. Which eliminates most of the on-line sources. Most do not reveal who made their belts, and most use their own part numbers. Most V-belts actually carry both the manufacturer's in-house number and the industry number. The spindle belt happened to get photographed with the industry number visible. See whether or not the motor belt carries a similar format number. The Atlas motor pulleys for the 9", 10" and 12" are all for 1/2" FHP belts so we know that it will start with "4L".
The casting part numbers are interesting. But are you sure that the spindle hanger (upper spindle bracket in your post) is D-21 instead of 9-21? All the way up through 1966, it is shown as 9-21 in the parts manuals for 10". 10-6 is the tailstock base, not the tailstock. However, I would have expected 10D-6. The tailstock itself will be either 10-5 or 10D-5. You'll find the number in the recessed front of the main casting. If you should happen to pull the compound slide off of the compound swivel, let us have that number, too. Should be 9-303 I think.