My personal preference is to grind the 312 main journals on the crankshaft down to the 292 size when given the option and here are some of the reasons I use to justify this.
1. The cost of the 312 main bearing sets are exorbitantly more expensive than the 292 main sets.
2. Bearing speed for the 292 mains is less thereby making the 292 main size more favorable for increased performance and/or efficiency with everything else being equal.
3. The main bolt spacing is an inherit block weakness for the larger main journals.
4. The larger journal mains do not tolerate as much main bolt torque as the smaller sized main bores on the 292’s.
5. The 312 main bearing sets are becoming increasingly more difficult to obtain in the various undersizes and especially in the name brand bearing sets.
6. It’s ultimately less expensive for a machine shop to cut the mains down on a 312 crankshaft versus boring a 292 block out to the 312 main bearing size. The bearing tang slots must also be put back into the journal bores if boring the main bores out to the larger size. When boring the mains to the larger size in the block, then a decision must be made early on to either leave the main seal at the 292 size or remachine the block to the larger 312 seal size.
7. Until recently, real main seal selection for the 312 was limited but Best Gasket has solved this issue.