With a mild cam, on a stock Ford distributor, there's no way in heck you'll need "manifold vacuum" for a vacuum advance. If you had a really lopey cam, very low idle vacuum, and a few other issues with cam timing or too-big-of-a-cam, you might be able to band-aid it (like I did) with manifold vacuum.

If your vacuum advance diaphragm was ever changed, chances are it's adjustable. You might need to go for more or less, depending on what's happening.

Running warm at idle could be a couple of things, nothing related to timing.

Do you have a fan shroud? Clutch fan? Is the fan big enough compared to the shroud? Is the fan right IN the shroud or is it sitting too far away from the shroud? Etc. etc.

As for mileage, a vacuum advance change MIGHT help that, but running on the highway, it won't do a heck of a lot. Initial and mid-range (mechanical) advance might do more for that.