First off, if it's cheap enough get the '73 no matter what.
As far as 'CUDAs go everything is interchangable because it all fits. if you want to put a '70 front end on '73 or vise versa that's no problem. Same with welding on a different tail panel, or bolting on doors and trunklids ect.
The real question is are you just trying to get the parts to make it road worthy or do you want it all correct and pristine.
I have a feeling your question is "Can I buy a '73 and make it look like a '70 ?"
The answer is yes but it is going to be very expensive...