Had a HUGE cam in a 400 with a Torker II and an 850 Q-Jet. Had a 4.5" vacuum signal at the rear carb port. Had same symptoms as you describe.

You must have a weak vacuum signal somewhere, leaking power brake booster, bad brake booster valve, vacuum leak at the base plate, carb to intake or bad float level adj.

With it becoming worse after it gets warm, metal and gasket material expands somewhat. Isuspect a leak at, near or within the booster/valve if equipped. As well, cars will idle higher when cold (choke) usually around 500 rpm's. If you have a leak, the higher RPM's when cold will offset the leak. RPM's induce vacuum. I suspect that your 500 RPM drop after warm is causing the problem. Try, just for [censored]'s and giggles, after car is warm, adjust idle to 1500 and drive it around. You will not stall or sputter.....Also higher RPM's increase voltage output. if low at idle, maybe you are getting a poor signal to that electronic distributor. Remove alternator and battery..take to Autozone or Murrays for a free load test. Check your battery charge at alternator and at battery..Mine reads 16.4 at alternator and 14 at battery..you will get a voltage drop with accessories. lower voltage at battery, under 12 is a sign that the regulator, lights, starter, distributor or heater fan (could be) poorly grounded or causing excessive draw.

Placing the car in neutral increases engine RPM which, in turn produces extra vacuum signal to replace the leak where ever or to supply sufficient neutral balance to the leak as to zero out the leak loss. You should be at 900-1000 RPM idle with that cam and an auto trans. 6-700 with a manual trans

Plug a vacuum gage in the rear port of carb. read in neutral and in drive after warms up. Post results. The main port in rear is the port for the vacuum to brake booster if power brake equipped. If power brake equipped, and signal is 15-17" you are good. If in drive, it dives, you don't have a booster or booster check valve issue. If not power brake equipped, you may have a leak somewhere if signal is below 15" with that cam. I have never checked a vacuum signal with that 067 grind, so maybe someone can check theirs and relate.