Just seeing your latest post so the below might be out of date. Would just say that the shifters on these cars are problematic so not surprised your car would start in drive. There is much advice on these blogs about how to resolve shifter issues. I don't go anywhere without a blocks of wood to chock the tires with if I have to park on a grade.
In the videos after startup and after 10 minutes the engine actually sounds pretty good. The 625
Holley isn't too big, the stock 390 can accommodate a 675 cfm
four barrel carb, so as long as the
carb is sound, you're not delivering too much gas at idle. With a high idle like that, it could be 1) an air leak (keep looking), or the throttle linkage isn't correct so with foot off the gas the throat of the
carb is open, or your distributor advance is set way to high. To address
carb linkage, try disconnecting the throttle linkage from the
carb and see how the engine idles. Re: advance, "spec" advance is 6 degrees BTDC. These 390 FE engines can take more advance than that, but initially make sure the advance is set to "spec".
By the way, your paint job on the valve covers looks great!