AC Delco Rapidfire #4 plugs in a slant six

You should remove the washers on an early engine with spark plug tubes. This puts the spark gap in the optimal position.

Most important to check the length of the plugs against your current ones, just to verify the Autozone book. You wouldn't want them hitting your pistons (turn engine over by hand first to verify, by pulling on fan belt). If they fit any slant engine, even 1980's, they will work.

If you have the original points ignition, or even the later Mopar ignition, set the plug gap at 0.035". If you have an after-market ignition that doesn't need the ballast resistor, or a CD or MSD ignition, you can set to 0.055" or whatever that manufacturer recommends. If it misses under WOT at low speed, decrease the gap a bit. I had to do that with my factory 2.4L Mopar engine.

This is an opportune time to change the rubber seals on the spark tubes. Wags term them "drool tubes". The seals are real cheap (rockauto, maybe Autozone). Down the road consider an HEI ignition if still running points. It is simple and cheap.