Weird starting issue

[Daddy Bartholomew]

1971 GMC 1/2 ton, 2wd, small block 400, 700R4, 3.73 posi. Same owner since 1982, brother-in-law owned it since 1975 or so. Bone stock when I got it, with a 350/350, 3.07 open spool gears.

After years of off and on tinkering, I finally swapped in a tilt column and a tach dash, and immediately afterward the truck won't start until I shift through the gears from Park to Drive and back. Then it fires right up.

Usually. Sometimes I have to repeat the process a few times.

It doesn't appear to be the neutral safety switch; the final position of the gear shifter hasn't changed by shifting it through the gears. It starts out in Park, ends up in Park. But it then cranks and starts perfectly.

I have no idea if it's related, but the instrument cluster now has a Tandem Lock light that comes on when I try to start it. I can get rid of the light (just haven't bothered), but didn't want to start fiddling till I had some idea what is happening.

Any ideas?