If you meant nanoseconds (ns) instead of microseconds (us) then, while your math is correct, the IIci does not require 40ns ROMs. It must have some wait states built into the memory access system.
When Gamba and I built a IIci ROM module we used either 90ns or 120ns chips. I can't remember which.
Forty or fifty nanosecond should be much more than fast enough. In fact, on the PCI PowerMacs 120ns Flash works while the Beige G3 needs 90ns. But none of these even more modern machines need 40 - 50ns memory for the ROMs.
So, I think your chips should work.
I can think of two possible issues you could be having. One of them is unlikely, as I don't think you'd made that kind of oversight, but just for completeness....
1) The pin out of the PLCC32 chips changed significantly between the 512Kb capacity and the 1Mb capacity. If your adapters are wired for the 512Kb capacity, they will not work with the larger capacity chips. I built my programmer adapters for 512Kb and below PLCC32, and 1Mb and above PLCC32 and they had to be wired to the DIP very differently.
So check your adapter's wiring if you didn't that into account when you bought the adapters.
2) Some (many?) of the later Flash chips have a Reset_ pin which must be tied high in order to read the chip. Check the datasheet for the flash you are trying to use. I know that a bunch of the Atmel chips have this. It's one of the big pains of dropping later Flash chips in place of ROM chips. You end up running wire wrap from the Vcc pin to the RESET pin.
When Gamba and I built a IIci ROM module we used either 90ns or 120ns chips. I can't remember which.
Forty or fifty nanosecond should be much more than fast enough. In fact, on the PCI PowerMacs 120ns Flash works while the Beige G3 needs 90ns. But none of these even more modern machines need 40 - 50ns memory for the ROMs.
So, I think your chips should work.
I can think of two possible issues you could be having. One of them is unlikely, as I don't think you'd made that kind of oversight, but just for completeness....
1) The pin out of the PLCC32 chips changed significantly between the 512Kb capacity and the 1Mb capacity. If your adapters are wired for the 512Kb capacity, they will not work with the larger capacity chips. I built my programmer adapters for 512Kb and below PLCC32, and 1Mb and above PLCC32 and they had to be wired to the DIP very differently.
So check your adapter's wiring if you didn't that into account when you bought the adapters.
2) Some (many?) of the later Flash chips have a Reset_ pin which must be tied high in order to read the chip. Check the datasheet for the flash you are trying to use. I know that a bunch of the Atmel chips have this. It's one of the big pains of dropping later Flash chips in place of ROM chips. You end up running wire wrap from the Vcc pin to the RESET pin.