The new antenna comes with various toothed spacer rings which can be stacked to accommodate various fender thicknesses. Unfortunately when using the OEM parts, all these spacers are too thick. They are also slightly too small of diameter to fit well in the OEM inner metal part.
The solution requires customizing a washer to use in place of the spacer rings provided. I started with a 9/16" inner diameter "Machine Spacer" found in a local Ace hardware. This fit over the new antenna shaft exactly so I didn't have to mess with the inner hole. Whatever you use, you want it to be about 1/16" thick so that the threads on the antenna shaft protrude the right distance on the outside of the car.
The outer diameter needed to be ground down significantly. I screwed a bolt into a board, where the bolt head was of a size that allowed the washer to spin freely. I then used an angle grinder on the outside of the washer while allowing the washer to spin freely. Leather glove on the washer-holding hand probably would be a good idea (my thumb was living dangerously).
One side of the washer then needs to be ground flat, as shown in the photo above.
A second problem is that the new antenna assembly is taller than OEM. When installed with the standard OEM inner metal piece, the bottom of the antenna box hits the sunroof motor and cable.
The solution is to mount the antenna rotated 180 degrees from normal which puts the bulkier part of the antenna away from the sunroof motor.
However, the OEM inner metal piece fits over the coax connector in such a way so that it can't rotate. Cutting part of it away, as shown in the photo, allows the antenna to be mounted at nearly any rotation. An angle grinder with a metal cutting wheel makes quick work of this as it's relatively soft pot metal.
Above photo shows how the customized pieces all fit together. I superglued the washer into the metal piece to keep it in place during assembly. I also put a little touchup paint on all cut edges to help slow corrosion.