As stated before there is an O-ring on each side of that transfer tube, the rear one is either pinched, broken, or it has hardened and no longer seals. Being that the carb has the nylon washers, it may also have reusable gaskets. I would take out the rear bowl and see if you have the reusable gaskets (they are usually blue), even if you have the more modern Holley gaskets that are brown/tan, they have a coating on them so that they do not stick, if they did not stick you can reuse them. When you remove the rear bowl you can remove the transfer tube and replace the O-rings, you need to put some oil on the O-rings so that they do not get pinched. You can buy just the O-rings:
https://www.summitracing.com/parts/...9doWv5NF_IDQ3E5ErBj_qOSZU1Z08UAYaApgNEALw_wcB
If the carb is running really rich, you probably have debris on a needle and seat, a bad needle and seat, or a float that is way too high and thus it cannot seal. Usually if you have any of these things going on you will have fuel coming out of your boosters at idle. Let the car idle and look down the barrels of the carb, you will probably see drops of fuel coming out of either the front or the rear boosters, if you do, you now know where your problem lies, either in your front or your rear bowl or maybe both (rare for it to be both). If you are lucky, you will have the leaking on the rear boosters, and when you remove the rear bowl to replace the O-ring you can look at the needle and seat and see if there is some debris there. Debris is usually the culprit if the car was running fine before you had any issues, so just remove the debris from the needle and seat, put your new O-rings in place and you are done. The second most common issue is just the float being too high, if its too high just adjust it down. A good rule of thumb on Holley carbs, for initial setup, is to have the floats be parallel with the bowl, you achieve this by turning the bowl upside down and just visually making the bottom of the bowl (since the bowl is upside down it is actually the roof of the bowl) and the float look parallel with each other. You adjust the float level from the outside by screwing in or out the needle and seat adjusting nut and screw. You do the final adjustment of the floats with the car running at idle, where you remove the side fuel bowl plugs and set the float to where the fuel is at the bottom of the threads of the screws and just barely leaks out. You can check to see if your needle and seat are sealing by blowing with your mouth into the opening where the transfer tube goes in (on the rear bowl) and holding the float closed against the needle and seat with very light pressure. If you blow into the bowl with the float closed and the air is escaping, and there is no debris on the needle and seat, you probably need a new needle and seat. They also do sell them separately. On the front bowl you need to blow into the fuel inlet and put a finger on the transfer tube hole to see if you have a good seal.
As stated by mrgmhale, you could also have a sticking or inoperable choke, or a ruptured power valve making the car run rich. On the video you put up the choke is closed, car may have just been cold, but if it was hot, that is why the car is running rich. In my experience a ruptured power valve is quite uncommon, but it does happen, especially if you had some issues with backfiring. I do not see any evidence of black soot on that carb, so I don't believe you had a bout with a backfiring engine.
