Wally. Welcome to the site.
If you can see the rear flange of the crankshaft while the engine is in the car, then the possibility exists that you can identify it as a 312. Any inspection or flywheel covers in that area will need to be removed to facilitate looking up at the rear crankshaft flange. With the harmonic damper placed so that the TDC mark is in the 6:30 o’clock position when looking from the front, then the cast in recess on the crankshaft flange will be pointing straight down or in the 6 o’clock position. The 312’s will have a 5/16” round protrusion or dot sticking up in this recessed area while the 272 or 292 engines will have a ¼” or smaller or no dot. The only nuance to this is some 312’s came through with no dot on the crankshaft flange as did many of the 292’s so if you’ve got the 5/16” dot, you do have the 312 but if you have no dot, then odds says it’s a 292 although could still be a 312. At that point, you’ll need to dig deeper into the engine and check for the ‘ECZ’ main caps or pull a head and check the stroke. If the bore is smaller than 3.800”, then it’s obviously also not a 312 but anything at 3.800” or larger can potentially be a bored out 292 and not necessarily a 312.
Here’s a link to another thread discussing crankshaft identification.
http://forums.y-blocksforever.com/FindPost8676.aspx