Guru ROM just needs 32 KB of EPROM space, so both 27C64 need to be replaced with 27C128s.
Pins 26 need to be connected to A13 from the address latch - make sure there's no connection to anything else. A13 comes in on pin 39 of the Zorro bus. Trace that signal to the bus buffer's input and see where its output runs to.
Would be easier with a schematic or if someone with an adapter could describe it... :-P
PS: just looked at the A590 schematics: there seems to be no buffering, so A13 should be available on numerous connections. On the A590 it's already correctly connected though.
