Just thought I'd post an update, one month after posting this thread. First of all, I discovered that shift knobs made for 86-92ish F-bodies (3rd gen Camaro/Firebird) will NOT work on the P1800 and 1800S cars. Although the thread itself is probably the same, the geometry of the threaded hole and the reverse lockout spring on the M41 shifter precludes compatibility. On the f-body knobs, the threads are toward the "top" of the hole, deeper inside the knobs themselves. When you try to thread the knob onto the M41 shifter, the reverse spring hits the top of the hole and prevents the threads from engaging. I.e., the knob just won't go down far enough--the reverse lockout mechanism is in the way. I found two possible solutions to this problem:

1. Obtain some sort of threaded insert to add to the f-body knobs, effectively lengthening the threads. These are very hard to find--I spent a lot of time looking. I've obtained what I think will work (it's actually a helicoil insert), but haven't tried it on the car yet.

2. Find a different knob! If you're looking for a nice, round black plastic knob, I found a $2.39 solution! McMaster-Carr, a massive nation-wide industrial supply depot, stocks tons of different knobs for machines and such, in various shapes, materials, and colors. The one I found that would work is McMaster-Carr part # 6046K29. It is made out of shiny black seamless Phenolic plastic--looks and feels exactly like a quality plastic shift knob costing 10 times as much! It is 1-15/16" in diameter, a nice size. The thread is 5/8"-18 (narrow pitch), which I found to be the correct thread for the earlier 1800's (virtually the same as 16-1.5 in metric, which has also been suggested). The best part is that the threaded hole is very deep--deep enough to work. I installed the knob on my '67 and, with the locknut removed, it works perfectly. It sits nice and low and provides great grip for my large hands (I'm 6'3" and, yes, my head touches the headliner). I am very happy with the results, and it cost less than a gallon of gas!!!