With that cam you should have between 18-20 degrees initial. it's 248@50

With the vacuum advance disconnected (hose plugged). Connect a vacuum gauge to the manifold and advance the initial timing to max vacuum less 1".

So add advance, check vacuum, you'll likely need to reduce the RPM via carburator as more advance will increase RPM. Keep adding timing and maintain your desired RPM (900-1000) until the vacuum doesn't increase. Back the timing down 1" of vacuum and that what your initial should be set to. If you find the starter dragging you may need to take out some timing.

Your total should be around 36, your initial is ~18-20 so you'll need to weld and file the distributor slots for 16 degrees mechanical advance (or purchase from FBO an adv plate limiter).

You likely get 12-14 inches of vacuum.

For the rpm not returning at idle, that could be the distributor, have you inspected it (remove the plate and take a look). Do the weights move freely?. Find someone with a distributor machine to set your mech advance, advance curve and check phasing.

I had the same cam in 440-6pak. Had Liteweight setup the carbs, there are adjustment screws under lead plugs in the outboard carbs that made a difference.

Good luck

I have a very similar setup and cam..
The vacuum is low..
Mine is about 8 inches
The Mopar HP distributor has 20 degrees (@ the crank) and 15 degrees initial making a total timing of 35 degrees. And my idle speed is about 1050 RPM, except my carb is a 770 holley.

Just my $0.02..

hi
where are you connecting the vac advance to ?

it must be hooked to the pas side metal tube on the center carb metering block .

a wrong connection could be the cause if it is returning to idle when not hooked up !