I was given a "Marksman" FP with no cartridge or converter.

Took it to Laywine's to try and get a converter for it. The Parker converter fitted nicely, but it was too long for the pen barrel.

The lady suggested if I was desperate, I could buy the converter and see if I could chop off the end to make it fit.

Has anyone ever done this?

You should try to check how the tip of the converter was made, and I would suggest trying to find one that you coud buy via internet that fits right instead of a cut converter...