Hey, dummy!
I can't remember which gasket I used (which wasn't that long ago)...dang pea-brain of mine. I think I used the larger tapered one.
Unscrew two of the screws on the gas cap base (opposite of each other). Screw the two brass thingies in.
Start threading the flush mount base in by hand (leave the cap installed). Carefully use a long screwdriver (or similar device) against the brass thingies to snug the gas cap base down.
Leaving the cap in, unscrew the brass thingies and but the screws back in.
You leave the cap in place to keep from dropping screws and such inside the tank.
Hope that helps.
