b and m megashifter nightmare

First off, you might be feeling the linkage traveling past park (which they do)
It is actually in park but the lever can travel farther.

Most aftermarket kits have a linkage arm that goes with their kit, so the geometry and travel is right for their shifter.

And while I was typing chryslerfat mentioned putting it all together in neutral.
A good way of doing it because the trans and linkage are both in their center of travel.

If you can figure out what it needs you might be able to mod your lever to fix the geometry issue, or you can contact the manufacturer and order the lever that is supposed to ship with that shifter.

I have a Hurst Promatic 2 and use a B&M cable for a completely different shifter because Hurst only has a 5 foot (WAY frickin long) cable and B&M makes a cable that fits and is 3 feet long.
Shifter issue's suck. :D