Idle Rpm High in Park, Low in Drive

Start with getting your initial timing set right and go from there. Anything else you do with that not set properly is just wasted time and effort.

I run MVA on both my Duster (modded 450-hp 360) and D200 pickup (stock 360) and it works great but you NEED to have everything else in the timing dialed in (initial, advance curve, total mech advance). It needs to have enough initial timing so when the vacuum drops and reduces some of the vacuum advance the idle speed doesn't get affected. So get it dialed with VA disconnected and capped so the RPM drop is minimal then experiment with ported vs. manifold vacuum to the advance can.