Thanks!
I chose a spot on the stalk that I thought looked good (it's around the three inch mark) and cut it there. I also chose the spot that would have a thick enough shaft left for the cap to be friction fitted. I guess it's really up to you where you chose to cut it according to your own taste.
Edit: I noticed looking at the pictures again that I made my cut right at the junction where the taper ends and the thin part of the stalk begins. That's easy enough to find
I got the cap at Lowe's. They are in the loose hardware section with bolts and screws etc. and come in bags of four pieces. There are four different sizes...unfortunately, the four sizes are also different colors with black being the smallest. The white caps are the next size up but then you would have to paint the cap to match. The black ones are very tight but they will go on if you work at it.
As for FM reception, I haven't been back to the car since I did the mod yesterday, I'll get back to you. I don't think it will affect it in the city at all however. Weaker, distant stations may not be as strong but local stations should be fine.
I'll take a picture of the antenna mounted as well.