I've made some changes to my program counter to incorporate a different counter chip. I found the 74LS593 counter. It's basically the same except it has a single bi-directional bus. This is handy because now I can tri-state my counter output when I want to do data memory access. I ended up completely reworking my block diagram to incorporate it. I needed to add a couple more chips too, but now I believe all of non-control chips are in place. The control logic might be a bit of a bear, but the datapath is nice and clean. I'm also putting together a real schematic, so this is my guide for that.