The WizNet 830MJ contains pretty much everything we need apart from a small amount of chip select logic on a single carrier board.
The RC2014 interface is extremely simple. A 74HCT138 generates a base chip select signal, a 74HCT32 qualifies the read/write signals to the WIZ830MJ and an LD1117 generates 3.3v to supply the WizNET.
The I/O interfaces are 5v tolerant so the only remaining issues to deal with are the timing, the reset and the voltage shifting. The reset relies upon the RC2014 backplane reset, which usually works but isn't perfect - it would be good to add a proper reset control. The timing is managed by using the W5300's ability to extend the usual 7ns data hold time. As that's in a writeable register we can write the hold timing even when read isn't working properly.
The voltage shifting is done by ignoring the problem. The low from the WizNET is near enough 0 and the high is near enough 3.3v. The RC2014 backplane runs HCT parts and the Z80 CPU expects TTL signal levels with a Vih of 2.2v so it all just works.
With some of the other processors level shifting would be needed - the 65C816 and friends for example would need level shifting on the data lines when reading.