Early A-body boosters used dual diaphragms to work with a small 7" diaphragm. They changed to ~10" single diaphragms in the 1970's. Clearing the valve cover in a V-8 is the main concern. If a slant-six, a 10" booster "should work" even in an early-A (1963-66 w/ narrower engine bay). I used a 1990's Intrepid booster on 1974 Dart standoff brackets in my 1965 Dart and just barely clear the aftermarket aluminum valve cover of my 273 engine. Indeed, I have to unbolt the booster to remove that valve cover. Best deal today is a new booster w/ brackets for ~$175 on ebay, using a GM booster and adapter. Haven't read that it doesn't clear an early V-8 valve cover.