383 Wont Idle with new to me 800cfm DP. Idles and runs ok with Edelbrock/AFB

Then what will help to start to narrow the issues:
Carb list number and revision.
The amount the transfer slots are now visible below the throttles.
Fuel level.

When you get a chance timing at 800 and some higher rpms.
However, my inclination here is still to return the carb and start with used old school one.
I think its going to need some reworking and that means you'll need to do it, along with a bunch of learning.
There's some weird carbs that are just bad to start with. No reason to knock your head against the wall with those.

and psst. A power valve leak is one thing, but an open PV is completely different.
The power valve opening just allows more fuel into the main well. The idle jet is downstream. There's a big hole (the main jet) and a small hole after that (the idle feed restriction). It doesn't matter if the big hole is even bigger. You can take the main jets out or open the PV and it shouldn't effect idle. At least when things are set up properly and sized right. It can make some effect when things are off - like the main jets are small and the IFRs are large. Even then, not a major effect like your dealing with.