Engine requires 20 degrees of initial to idle well...

she's getting there LOL, I just switched my intake manifold out and with the new manifold installed my vacuum gauge was fluctuating a bit at idle (just like yours was doing, hunting a bit). Turned out the idle screws needed some adjustments after I checked the initial and total timing. Made a big difference.

Is there a difference in the idle screw position between this new carb and the old one? Also, did you end up limiting the fuel pressure to 5-5.5 psi?