OK, so I know nothing about 914 gauges other than that they're pretty big (I think they're bigger than 356 gauges, or at least the same size, but what the hell do I know??)
What I have in hand is a 912T gauge set and can actually measure those puppies and here's what I found:
All three gauges are mechanically the same:
The gauge can is 3-7/8" dia. while the rubber bezel is a tapered affair that starts at about 4" dia. on the slim (inside) end and varies out to about 4-1/8" where it meets the dash hole. The lip of the bezel extends another 1/8" to become a lip into the dash gauge hood, which accepts all three gauges as a unit. This (admittedly, el-cheapo) bezel takes the place of the nice, chrome bezel on the 356 and the black trim ring on the 914. It is a slip-on from the cockpit side of the gauge. Removing the bezel leaves a naked, ugly-looking gauge/can with nothing to keep it anchored to the dash. There are places where anchor posts could be installed on the back of the gauges, but they relied, instead, on a pressure fit of the el-cheapo bezel into the tapered gauge hole in the gauge cluster mounting plastic hood with an alignment tooth in the bezel to keep them straight. This was the el-cheapo 1970's, remember??
As a post-script, I'm using 912T gauges on my current build to overcome the problem of small-ish steering wheel versus big gauges and then having the wheel cut off some of the gauge. With a bit smaller gauges I can fit them inside of the diameter of the wheel and actually see everything on the gauges. Yes, I know this wasn't a problem with original Speedsters because they had those HUGE steering wheels, but they're not for me....