I installed a '74 dash into my '72, and it was an extremely labor-intensive job. I have about 40 hours invested so far (just in the dash installation--not counting removal of the previous stuff) and it's not quite finished. I was fortunate in that I also had a '74 heater box, which is different from the early-dash box. It physically bolts in the same place (thankfully) but the outputs are all different. You could probably modify an early heater box to duct the air to the new locations without much difficulty.
Be advised that you can't use a '74 dash with the early-style center console and instrument panel. Surprise! The vertical dimensions of the two interiors is different, and it's my understanding that if you try to use a late dash with an early center section, you'll be left with a gap about one inch high at the bottom of the dash.
That said, it can probably be overcome....
Also, the late and early dashboards are physically mounted to the car in a completely different fashion, requiring fabrication of mounts on the car to accept the mounting tabs on the dash.
Hall Pantera sells a beautiful ready-to-go cover that slips right over your wiped out single-pod dash and renders it looking like new. Nice piece!
Finally, I'd ask what is inspiring you to make the change at all? If you simply need to install *A* dashboard, the early two-hole dashboards are extremely plentiful and have virtually no value; I'm sure you could buy a beautiful one for under $100. I *gave* mine away for nothing.....