Old memories and some digging up pointed to some differences apart from the address bus width. They have a somewhat different way of handling bus arbitration. The 68EC020 doesn't have a bus grant acknowledge signal, whereas the 68020 does. I really can't remember all the subtleties of the 68k series, but I think this may explain why the 68020 appears to get stuck (assuming that in the package you use, they are pin-to-pin compatible otherwise).
Look for 'BUS GRANT ACKNOWLEDGE' in the datasheet, and go from there. May be a clue as to what's wrong. It might just be a matter of pulling up this pin on the 68020 and not leave it floating, but I'm not sure. You may have to add some glue logic. Much too old memories. You have to go from there. Good luck.