Not finding needle valves readily available was probably a good thing. I probably would have ordered them, however, I went out to the garage and took the float cover off again and inspected everything again. This time I used about 10 psi of compressed air and checked to sealing. The float was turning quite a bit angled in order to get it to seal. That doesn't make sense to me. I don't get why the floats aren't more level at the point where the needle seals against the seat. In this case, the bottom edge of the float appeared to be crossing over where the side of the float bowl would be (if the cover was on the bowl). Perhaps the side of the float was interfering with the side of the float bowl and not able to rise more to seal the needle against the seat.
Though I used the clear tube method to set float levels previously, I repeated that process tonight with this carburetor bowl, and again set the float level so gas came up to a mark I set at 20 mm from the bottom edge of the float cover. Is 20 mm from the bottom of the lid is correct?
The needle and seat is working properly again now. I took the car for a another brief 3/4 mile drive. It was not fully warmed up, but it ran a bit better than before at 2.5 turns out on the mixture screws. Assuming I don't run into another issue, I will get it up to temp and try another brief road test tomorrow to see if I will be able to get the mixture close by backing out the mixture nuts a bit more.
Perhaps I will need to switch to different needles. I don't know.