IMO as well as many, many others... Do not get a harness bar.
If you were ever to be wearing it and roll the vehicle, you'd be F'd. The stock 3pt allows for collapse and lets your body get out of the way of a roof caving in... Do yourself a favor, if you really really want to run a harness, run at the least a 4pt bar setup..
I have a custom made bar that attaches to where the stock seat belts go. It is an autocross car so I'm not terribly worried about a roll-over severe enough to cave in the roof.
I run a 4 point roll bar, cant use the back seat but if you run a harness bar you dont want anyone back there as well, in an accident they could fly forward and beat themselves fairly badly against the bar.