When I was fitting flag mirrors several years ago, I found the following picture useful: it shows that 911 mirror bases are the ones to use on 914s BUT the bases are interchangeable: the flag bit of the mirror is identical, no matter what car they came off. There are 2 sorts, early and late, but the difference is the method of attaching the mirror glass and there is no physical difference once assembled.
Click to view attachmentOther points to consider are:
The bases have to be re-profiled, either more or less, depending which side. The one on the driver's side fits further down the door and getting it to sit correctly on the 914 door is marginal. It MIGHT pay to get a RHD left-side 911 base and use that instead: it would move the mirror back & up, the increased angle MIGHT be OK, and the mirror would sit better and not be so prominent. The only problem is that fitting them involves cutting holes in the door so experimentation is out. (Or, a supply of useable, scrap, doors).
Seemingly no-one has used 1974 911 mirrors. They would seem to suit the car, and they're of the correct sort of vintage, so why not?