You hit the nail on the head, "no ones seems too interested in the old EFI setups!".
What you have already changed is good analysis of the problem.
There are a couple of things left which may be the cause of rich running.
1. The Air Flow Meter.
This is a moving vane type device which after all these years may be well overdue for a good clean and test.
2. Cold start valve.
This is as the name implies only use during cold starts but may be leaking fuel past the valve ruining the mixture.
You could clean this fellow up as well and even disconnect it electrically and see if the engine behaves any differently.
I gather you have a Manual of some sort?
I have a Haynes Manual which seems to be very comprehensive for the 280ZX EFI engine, let me know if you need a scan
P.S.
It may be time to upgrade to an aftermarket ECU which would do away with 50% of all that old outdated troublesome technology
