you need to find out why you only have 10" of vaccum.do you have a wild cam,vaccum leak,you should have a lot more for a stock 74 duster.assumming its stock,check your hose going to the booster,could be cracked
he meations a different cam in his first post; I'm assuming its something quite radical with no idle quality's, if not he has a very bad engine vaccum leak or something