The timing of the Z80 /RD signal is very tight, so the choice of NOR gate IC that you put at U2 is critical. Choose a fast part such as a 74'''F'''02. At least one manufacturer, Misubishi, makes a fast enough 74'''LS'''02 and that is what I used in my prototype. It has a propagation delay of 6-15ns, and the typical delay is somewhere in between, say 9ns. This appears to be the lowest speed the design will allow, although I have only tested with a few '02 variants. If in doubt, check the manufacturer's datasheet and don't buy anything with an average propagation delay larger than say 9ns. I've listed the delay times for several different '02 logic chips below. It's quite surprising what the propagation time range is for parts that are supposedly "standard".
Note: In the table, "Likely" means "my best guess based on the published proapgation delay times". Avoid anything that is "Not comaptiblecompatible".
'''74??02 Propagation times'''