Starting in 1980, G-bodies did not use a neutral safety switch. Instead, a mechanical interlock in the steering column only allowed the key cylinder to engage the starter in Park and Neutral.
There is a neutral safety/back up light switch is mounted on the column, but only the back up light connections are used.
To use the B & M shifter, you could leave the linkage to the column shifter in place and remove the shifter handle from the column. This will leave the mechanical interlock in place, maintain the factory reverse light switch, and maintain the dash PRNDL display.
If not, you need to run jumper leads from the back up light switch in the column to the shifter back up light connections, and then cut into the starter circuit and run jumpers to the B & M neutral safety switch.
Also, you need to lock the steering column shifter lever sleeve in the park or neutral position, or the mechanical interlock will keep you from starting the engine.
What Dave said. I put a B&M shifter into my 84, moved the reverse light lines to the shifter and cut into the starter line so the shifter controls it. Collar on the steering column just stays in the park position.