Something radically wrong. if you can idle at 675 rpm, you should have more vac than 10.5"; similarly 750 rpm in Park would expect more than 13". Maybe faulty vac gauge?
Surging @ 38 mph could well be on the idle cct, NOT the main cct. Back out the idle mixture screws a full turn & re-test. If better or fixed, the idle tubes in the pri boosters need enlarging.
Unless you find the optimum idle timing that I outlined [ read the link ], then you are just p*ssing in the wind....& so are the rest of us that are trying to help you.
You are quoting pages from the Edel manual. Did you read the advice about using as much idle timing as possible, & using vac connected to man vacuum as a method of doing it?