They are rare in any speed.

Maybe some fake 10ns ones would work in this use?

The 64Kx8 ones are 32-pin. If you look up the 32Kx8 (28-pin) pinout you will see there are no spare NC pins to use as an additional address line.

Ok. The manual I'm reading shows 28 pin 64Kx8. Any idea if this was erroneous? They literally don't think people use more than an addressable range afforded by 256K or 32 MB FPM SIMMs. I know about boards made by manufacturer with fake SRAM. So, this could have been fake specifications that suggest a relation between 512K Cache and 28 pin ICs.

Your sockets are 32-pin (we can see it in the picture)--right now only 28 pins are filled. Your manual calls for eight 64kx8 chips (for 512K) or eight 128Kx8 chips (for 1024K). Either way, the tag ram stays as 32Kx8. That's why one of the nine sockets is too small to hold the larger chips. There are quite affordable ISSI 1024Kx8 chips on ebay but you need a TL866-II or something else to test them as around 10-20% are bad.