After some breadboarding and a late night soldering session I have the first bit of working hardware. This board contains the program counter, address buffer, and instruction decoding circuitry. I will eventually replace this board with a proper PCB - this construction method is fine until you get a loose connection, and then it's a huge pain. It's fine for now though and it allows me to work on the other parts of the system.

For the program counter and address buffer, I'm using 4x 74LS469 - a synchronously presettable 8-bit up/down counter with tristate outputs. Pretty much the ideal counter IC - I'm not aware of any other counter that has all the features I want in one IC. None of the 4-bit counters have tristate outputs, so you need double the number of chips plus a couple of octal buffers. The '590 is 8 bits wide with an output enable, but it's not presettable. The '593 is presettable, but only through the single input/output port.

The '469 does have a couple of issues though. Firstly it's obsolete and quite hard to find - there's a few on eBay. Secondly, for some reason I don't understand, they each consume about 100 mA (the datasheet says 120 mA typ), and get pretty hot! So maybe I'll replace them with something else when I do the PCB.