The neutral safety "switch" on most "later" model steering columns (those with the ignition key on the column) is actually a mechanical interlock that prevents the key from being turned into the "start" position in any gear but park and neutral.
I've converted several of these to use floor-mounted ratchet shifters and I've had to fix them in the park position and use the park/neutral switch on the floor shifter to interrupt continuity in the purple wire coming from the start switch in the column when not in park or neutral.
Other than those 2 methods, I'm not familiar with an easy way to do this on a TH350...unless you wanted to get creative and use something like a brake light switch on a bracket near the transmission shift arm. This would allow starting in Park, but not neutral. Something to consider, though.
