Chris,
That would take a lot of filing, and doing it carefully would take a lot of time. I may have to resort to that in the end, but I am going to explore other solutions.
The specs for the FT-8800 say that the connector is "SO-239 (USA version).” I am wondering whether the diameter of the socket of the "USA version" is smaller, and if so where I can find the corresponding (with a thinner centre pin) version of the PL-259, and under what name.