AT-D578UVIII Plus

This is a very popular radio.  This is good news as there are lots of people out there who have already done the work of figuring out how these radios work and how to modify them.  Jason Reilly has put together a fantastic page packed full of info on modding these radios.  I will link to the section covering band selection, but I suggest you read over the entire page if you have one of these radios.

http://members.optuszoo.com.au/jason.reilly1/578mods.htm#SelectingBands

This is another case of don't reinvent the wheel.  Brian Wall W1BKW has put together a nice document covering how to connect the AT-D578 to the SignaLink.  You can find it here.