Depending on if you want absolutely correct or pretty close, you have a few options.
The Monte dash carrier will work if it's in good shape. You will need either a new dash harness, or be willing to do a little re-wiring. You then buy all the gauges separately and a new circuit board for the back of the cluster. You can also buy a gauge conversion kit from MuscleFactory, which has what you need to add gauges into the dummy light spots (and includes the jumper wires, diagram, and instructions). You should be able to re-use most of the other trinkets from the sweep dash. The plastic under dash stuff might not work directly, like the ashtrays are different I believe, so keep all the Monte plastic stuff.
Pulling out your old dash is pretty easy, and that's the time to replace your heater core if you're gonna do it at all. Same with under dash insulation/firewall pad, etc.
I have a '72, got a dash from a '72 Monte too. I gutted everything in front of my seats, and am slowly putting it all back together now.
I bought a new carrier, MuscleFactory kit, and have been accumulating some new switches and stuff. Gonna finish my swap before Easter if all goes well.