I just got done the same project. Yes, the dash harness is different for the round ss gauges. You can find one on E-bay. I think Painless sells a new one complete with labeled connectors.

 

http://www.americanautowire.com/ http://www.wiringharness.com/
Both of the above links have factory fit harnesses, complete with factory style connectors, basically plug and play.

 

Just to clarify the sweep dash harness is very different than the round SS style harness. The sweep dash harness can be modified to work with the round gauges but on a scale of 1-5 in difficulty it is a 5 for a novice. The SS style idiot light harness is different than the SS style tach/gauges harness but can easily be modified to work with tach/gagues.