As far as I know. Replacing the AFM means replacing the ECU and most or all of the other stuff that the AFM utilizes (cause the AFM is programmed to react to certain inputs). While there are other options, the simplest and least overall cost replacement that I know of is an SDS unit that will keep the distributor.
I replaced my whole 280ZX ECU and AFM with one from a '87 300ZXT. All told, I spent about $150, including ECU, MAF, 300ZX distibutor disc, 300ZX ignition coil and trigger and miscellaneous wiring. Wiring it took lots of time, and I had it tuned pretty well for $25. Only drawback is I have no functioning knock sensor anymore - I think the L28 with mechanical rockers is too noisy for the 300ZX knock circuit.
Are you sure you need to change the AFM out? They are good for about 450hp, a little more if you port them out. I would go with the 300 setup if it was me. The reason you can't just swapp the AFM out for air mass is they read opposite. One reads high at an idle to low at full speed the other opposite. Put the mass air on your 280 and it will idle real rich if at all, cruz ok then go lean the more you get on it. Not good. Hope this helps,