Simple question, where did you get the ROM? I ask because if it didn't come from Vesalia or Amigakit (which means a "home brew") then the EPROM speed (using a 27C400) may be too slow to work -- they need to be around 125ns or faster. Above 150ns seem never to work. As stated before in the thread, an A500, A600, and A2000 are interchangeable, but no others; these are single chips (68000 based) unlike the dual chips (68020 or faster) in the A1200, A3000, and A4000's.
Also if the chips came from a certain auction site, they may not even be programmed right. Better to get a real ROM from a real company.