Thinking "Out of the box" here. How radical is your camshaft? Mechanical or hydraulic?? If mechanical, back the lash off a couple of thousandths. With a hydraulic, go zero lash and only an 1/8 turn past! It has the effect of dialing a little retard into the camshaft with little influence on performance, except for WOT. This MIGHT get your vacuum up at a slightly lower idle and give you more leeway for adjustment.
You can go in and retard the camshaft a degree or two at a later date if this works.
