A Teenager and His Swinger

Well guys I have a choice. I have a complete 1974 Dart Sport dash with every bit of wiring. I don't know if I should take out the old dash and put this one in, or just put in the under dash wiring harness in the one already in the car (got the harness with the car). If I keed the dash in it, I'll have to rewire it and find a new wood grain piece for the heater switches which wouldn't be hard. Or I can repaint the new dash, and just bolt it in. The big issue is that I really didn't want to pull the windshield out until I had a new one. It's got a few bad spots in it that look bad. What should I do? I might be leaving details out so ask away.