312 is the match set. youll find the same number on the od of the ring gear. that keeps the pair together.
2.261 is the true pinion depth measured from perpendicular of the center of the carrier to that surface. I wanna say its a -4.
thats the thing about setting up a blind axle. that is one without gears in it. you have to make an edjucated guess on the correct pinion depth and backlash. while there are standard specs, good luck finding them. I caught alot of things that I either read in a mag or something, and tried them with poor results.
if your taking a geaset out, then use its shims as a starting point. most of the time I only need to adjust the backlash and maybe a few thou in the pinion depth.
email me direct or even give me a call and I can help you through the install.
usually the base starting point for a Dana 30 is the .030 slinger pressed on the pinion, the .020 baffle under the bearing cup and another .030-.032 shim under the bearing cup.