What I did..
The tip rib should be fully riveted. The tip is only attached to the overhang of the skin (approx 1/2"-5/8" if I recall). If your skins were trimmed even with the rib flange, you may have to get creative. The spar stub that sticks out has to be trimmed and I did this with everything riveted. Slide a piece of thin steel (a putty knife works well) between the spar and skin, then trim the spar stub top and bottom at an angle of 45 degrees or so with a cut-off wheel to allow the wingtip to slide in. My spar stub protrudes about 1" long outboard of the rib. If you are still on the fence about rivets vs. screws to hold the tip on, I would recommend the nutplates/screws. Takes a little longer, but its nice to be able to remove them if needed down the road.
8A7 / Advance NC