You can set it with a timing light if you like but the vacuum gage a good ear and some patience will work much better as OldSub said the vacuum gage method is based on what works. Also keep in mined that the fuel quality when these old trucks were made was not good so today the same truck could benefit from a little more advance in the timing.
You may want to get it where it runs best with the vacuum gage and ear then put the timing light on it and record the numbers for next time and wile you have the light on it if you have a tach you can also check the advance curve and know that it is good also, the timing light has it‘s place but first you must have good references without them it is merely a flashy thing.
