I'm a big fan of mods and I have a Musicman pickup that I've been meaning to route for a 50's P Bass. As far as cutting into the Reggie, you could do it. I did for an experiment.
This is my Reggie with a jazz bass in the neck position and a reissued maple neck. The bass is back to normal now but I wanted to hear some things. http://www.youtube.com/watch?v=2XM2r...e=results_main
If you don't have extra bread to spend, let it go so that you can get most of your investment back and build something from scratch. It's not difficult these days.
Hope that helps.
All the best,