Ok, so I've got it all installed nicely. I ended up putting it above the speaker like I originally thought. I had to do a little surgery with one of the case screw posts to get it nice and even in there, but this isn't a big deal. Looking at it from the outside, you'd think the SK-1 came with MIDI stock, unless you happened to flip it over and peer inside one particular screw hole and notice the missing screw
Much better than removing the speaker like the old kits.
I do notice a bit of noise now that it's all installed. I'm not too bugged by it, since the SK-1 is pretty noisy and lo-fi by nature, but this is definitely related to the MIDI mod because I hear a faint little buzz whenever I send a note on or note off message. I assume it's a shielding issue, since I added a bunch of new wires, but any chance someone knows off hand what connection is most likely to be causing this interference? I have a gate set up in my DAW, tailored to the SK-1 (which would be necessary no matter what, since again, SK-1s were never quiet from the factory) that minimizes the problem, but all the same, if I can open it back up and shorten a cable run or add some shielding and have it fix the problem, I'll do it.