I figured I would change the lamp this morning.
When I changed out to halogen about a year ago, it was hard to tell how bright the halogen headlamps were until I got out on the road at night. I did have an old lamp and a halogen in at the same time (when I was first checking on the replacement of the burned out old lamp) and they do look different.
I'd change them both and see what happens. In my case, it was easy to tell the difference.
I don't think it would be the harness (That's the one you changed out a while back, right?), because the headlights are wired in series driver's side to passenger's. If there is something wrong, I'd check the driver's side ground wire first (There is a ground wire for each.); and then, the socket connection. I do recall having to work the connection a bit to get the first new lamp to connect period.
BTW, I asked one of the guys at Classic Muscle the other day about a dustcover for your powerglide (Did notice you were thinking about upgrading there, too.), and he said they hadn't been able to get those for several years, so they discontinued it from their catalogue. Sorry!