As far as I can tell, this chip is not used in electronics more recent than Gen3. You could save yourself a lot of pain by switching to something more recent.
Failing that, there was a Gen3+ board which replaced the extruder controller with a daughter card that mounts to the sanguino header on the main board. Its built to drive a modern stepper extruder, which the original extruder controlled can only run in full step mode, and it can run a heated bed which I don't think the basic Gen3 can. This will also allow you to use modern firmware.
Failing that, there was a Gen3+ board which replaced the extruder controller with a daughter card that mounts to the sanguino header on the main board. Its built to drive a modern stepper extruder, which the original extruder controlled can only run in full step mode, and it can run a heated bed which I don't think the basic Gen3 can. This will also allow you to use modern firmware.