Paul.  It use to be a common practice to put a 350 scrub crankshaft in a 400 scrub block which required either a special production ‘thick’main bearing set or the use of bearing spacers to allow the smaller main journals to work in the larger main bore block. This was the old 377 cubic inch combination that was touted for its big cylinder bores thus freeing up some head flow. This combination was doable in part because the rear main seal diameters were similar.  The aftermarket now provides a small journal 400 cylinder sized block which has greatly simplified that particular combination.

Doing the same for the 312 block using a 292 crankshaft isn’t feasible in part due to the rear main seal diameters between the two engines being so dissimilar.  2.625” rear seal diameters for the 272/292 cranks and 2.750” seal diameters for the 312 cranks.  I am ending up with an abundance of 312 blocks without crankshafts so I can see where this is going which is simply the need to use up those 312 blocks.  The problem with main bearing spacers besides the fit being critical is the heat dissipation being compromised which in turn creates the potential for a premature bearing failure.  While I can see a solution for putting a 292 crankshaft in a 312 block, it is cost prohibitive considering the number of 292 blocks that are still available to work with.