Do I have to modify the harness or how it sits in the dash?
Both. You also need to modify the door panels for them to be able to close.
Your going to need:
The whole dash with the dash harness.
steering wheel coverings
kick panels
Heater core
Dash Mounts
Fusebox
Center console/Armrest AND brackets.
Those are just the tip of the iceberg. You are going to need fab skills to figure out how to cover the gaps, make everything fit right, brackets to line up, etc.
Not just a plug and play. It is a complete modification and I have seen MANY of them look like crap. It is kind of like dropping in an S2K cluster. If you don't take your time and do it right it looks like garbage and you might as well not waste your time or money.
GOOGLE it. There are a bunch of write ups out there. This is the first one that came up under a 1 second search.
http://www.honda-tech.com/showthread.php?t=2447292