The extreme undersize connecting rod bearings for the 312 engines are easier to find as the same bearings are used on the 239, 256, 272, and 292 engines.  The 312 main bearings will take some searching as they are specific to the 312 engines and are larger in diameter than the other Ford Y engines.  I typically take those 312 crankshafts that are heavily worn on the main journals and turn them down to the standard size 292 mains and use them in the 272/292 blocks.  Heavily worn connecting rod journals can be turned down to 2.100" or 2.000" and then used with a different connecting rod.
 
Here is a set of 0.030" main bearings currently on EBay.
Federal Mogul 963M30 0.030" Main Engine Bearings Set 1956-1960 Ford 312 Y-block | eBay

Lorena, Texas (South of Waco)

I have a 312 crank that is pretty rough.  It will probably need to go .040 on the rods, maybe more.  Don't think the mains will clean up at .020 or even .030.  Is it trash?  Have not taken it to the machine shop yet.  I don't want to waste money on it if there are no bearings available.

Paul