You're absolutely right, SH. The same applies to the address bus. I can tristate these lines by reverting them to inputs.
Your mentioning of this has just caused an alarm bell to go off in my head
I've currently assigned Port B to low address and Port D to data. I think I must swap these roles because I'm pretty certain Port B is the only one that has TTL level compatible inputs, which is going to be essential for this project.