Well, if you have the EPROM /CS always selected and send the rom enable line from your decoder to /OE of the EPROM you can use 120 ns parts. The reason you don't have to subtract the decode time (usually) is that the 120 ns is the minimum time the address lines and /CS need to be setup. Usually the min time of /OE to valid output is fairly less than the chip rated time. So as long as your decoder doesn't take an unreasonable amount of time ... you'll only need to buy 120 ns parts.
> If you're desparate to test whether it will work you might try
>disassembling the SD3 code and finding the spot where it switches to
>fastrom and NOP that instruction out (hopefully it only switches once).
>It usually happens very near the beginning of the startup sequence.
I'd at least try this.
the code you're looking for is something like this:
They may use another byte besides $01, because all the really matters is that bit0 is set (as far as I know). So, once you find that ... change that one byte to $00 and it might work. But, the game might be jumpy/skip at points because the CPU isn't running fast enough.