Idle drop by 500 rpm?

I tend to agree, but until it's eliminated as a suspect, I'll be sumbitch if we'll ever know. It's called diagnosis. You eliminate one system at a time. I've been doing diagnostic work on carbureted and distributed engines for over 40 years. There's a procedure. I tried my damndest to outline it. **** it.
Easy big Boy, easy.
The OP is never going to get the engine to idle at 450 rpm in drive regardless of what you or anyone else recommends.
Keep up the good work and remember your blood pressure.
Stay Kool
Jim K