AC Delco Rapidfire #4 plugs in a slant six

You may want to check with Autozone, but I am pretty sure the Rapidfire are not meant to be gapped. They are supposed to come preset at the proper gap, and if they are the right ones for your car, they are probably already there.

If you have a flat plug mount, yes, you should use the crush washer. If you have a tapered plug mount, they are the wrong plugs.