You are correct. A tap is used to cut inside threads, like on a nut, or in your case the wing mount blocks. To cut threads on the outside, like on a bold, you would use a die. You can get these at most hardware chain stores. You will be asking for a 1/4 20 tap and tap handle. If you have not yet drilled your mounting holes, this is how I would recommend doing it. If you have a drill press, or a friend who has a drill press, drill a hole of the recommended diameter into a small block of wood. Say, something in the order of 2" x 2" x 1 1/2". You can use this block as a guide to keep the drill at the proper angle when you drill your wing mounts. Once the hole is drilled, tap the threads in the mount blocks. You can "harden" the threads by putting some thin CA into the threads. If you use the thin CA, after it dries, run the tap through again to clean the threads.
If you haven't finalized your Monocote order, you might want to consider Ultracoat. It works at a lower temp. and a lot of people find it easier to work with. The covering shouldn't affect the fit of your wing to the saddle, it is very thin.
On my 4*, I didn't add wheel pants. I used the Robart wheels with the solid hubcaps and painted them to match my color scheme.
A couple of things I found when I maidened my 4 * that I would suggest. Set your ailerons so their bottoms are flush with the bottom of the wings when in the neutral position. Also, be conservative with your throws. Better to have a little less throw than too much.
The plane can be a little squirly on the runway. To help this I did 2 things. I used the next size up tailwheel, and I added a little toe-in on the main gears.

Good luck!

Scott