Hi Steely! I found it was actually relatively easy to install the wiring. The real secret is making up a good wiring harness. This all depends on your wiring and soldering skills. I'd also suggest upgrading the capacitors to some Sprague Orange Drop ones, primarily because they have a tighter tolerance than the kit ones and they have longer, stronger legs (Gavin used them in the post linked to above). I'd also suggest buying some thicker wire than the thin very pliable wire that comes with the kit. I'd also suggest getting hold of some heat-shrink tubing, which both provides additional strength and prevent nd exposed metal parts from shorting out.
Make the wiring harness up using a piece of cardboard (Gavin used some perspex but cardboard works just as well as long as you poke holes in the right place - I transferred them over from a piece of paper I first used to poke holes in). Also mark on the F hole position, as you want to build your harness in a U-shape around it so that the wires don't show through the F-hole. Obviously you can't connect the pickup wires or the bridge earth wire at this point, but you can do all the other connections with the pots and switches poked through the cardboard and held in place with the appropriate nuts. Just check your hole orientation as you need to work as if you are looking at the underside of the top.
Don't make the wires muck longer than they really need to be. You need some wriggle room when fitting them, so make them just a bit longer than necessary. Put heat shrink over any exposed wires (e.g. the capacitor legs). Once wired, I wrap some insulating tape round the inter-pot wires to keep them together and create a semi-rigid arrangement. This helps keep the pots and switches all facing the same way.
You're then ready to transfer the harness to the top of the guitar (protect the finish with masking tape or similar) where you can wire up the pickups and bridge ground wire. Don't shorten the pickup leads as you'll probably want to run the bridge pickup round the neck-end of the f-hole to avoid being seen and the neck pickup lead along the string side of the f-hole.
The pots and switches will generally end up sticking up quite proud of the top if you don't use any washers to reduce the amount of shaft that sticks up through the hole. I'd use some callipers to measure the thickness of the top (normally about 5mm) to see how much sticks up. If you think it looks too much, then you can get some appropriately sized washers to fit on the shafts first to reduce the amount poking through. I'd stick them in place with some general purpose clear glue (paper/card type) as its quick drying and goo enough to hold them in place until the nuts are all done up.
With this 335-style guitar, (unless you have really sausage-like fingers that won't fit in the f-hole) it's really easy to poke almost all the pots and switches up through the holes using only your fingers - providing the wiring harness provides enough strength to keep everything pointing the right way. If it's all loose and flopping about, it's easier to use the tube method for everything (or you can use cotton through the holes and tied on to the shafts if you don't have any tube). The only thing that really needs to be pulled through is the jack socket. You can use a tube, or you can use some string pushed through the front of the socket, which is then tied into a knot big enough so that you can pull on it without the knot slipping trough the hole in the socket. I normally attach a small piece of plasticine or blu-tac to the string to weight it before pushing it through the hole for the jack in the body so that it pulls through the hole (guitar body held vertical) and can be pulled through the f-hole with some thin nosed pliers or tweezers.
Once the jack socket pull method is in place, then its time to feed the components into the f-hole, jack socket first.
Once all is inside, pull on the jack socket and it should appear in it's hole (you may need to jiggle it around with a thin screwdriver to get it to line up enough to pull through the hole). You may need to move the pots and switch around through the f-hole so that they are underneath their respective holes, but once that is done, then holding the string or tube tight, then slip the washer and nut over the string/tube, and tighten up.
You should now be able to simply push the switch and pots up through their holes using your fingers, with some help from some thin-nosed pliers or a small screwdriver. Washer + nut on each one and tighten. With the first one secured, it will lift the nearby pots/switch up as well, making it even easier to do the rest.
If you used string to pull the jack socket through, you can simply cut the string, push the knot into the body and a few shakes with the body inverted should see it fall out of the f-hole.
I've seen this method talked through by several different luthiers in guitar magazines and it was my first chance to try it. It took me about 2 minutes to get everything in place and do the nuts up. I was most impressed. But it does rely on a firm wiring harness, otherwise the pots and switches will simply fall over inside the body, making it much harder, or even impossible, to use this method.