The files in the folder referenced are 636KB each as hex. As binary, that should be around 512KBytes each, or 4 megabits.

I guess you could do the math on whether your two chips hold 8 megabits each.

I don't have an 859X but I am a computer guy working backwards from my phone.

I just saw an email pop up while writing this about 4x27c020. I think you’ll find the newer firmware is larger than the old and you’ll need 27c040 parts.

Being the Motorola processor begins its power up by reading from the bottom of memory address range and ram typically gets located at the top, this avoids having to remap address lines, but it also facilitates swapping larger EPROMs in so long as the sockets have enough address lines to address the entire chip.