really you need a head unit that is compatible with the Mach 460 system (it's a low ohm system), talk to Crutchfield for best results
Sounds like a really good idea. Another option is to get a few line-drop converters and run them from the RCA pre-outs on your new head unit to where your speaker wires tie into the harness you bought. RCAs put out a less powerful signal. Don't give up!