A stock or modified 312 is a very god running engine.  There should be no reliability differences as they are the same basic engine. An with a modern rebuild there are olmost no relibility problems with a mild Y-block.

The only down falls of a 312 are: possible cracked block, the main bearings cost more, and it is harder to get a good rear main seal than a 292.  The 312 should have the better flowing cylinder heads and much better compression ratio.

The 256 would be harder to find pistons and rings for, super low compression, horrible cylinder heads, and most of all, it is only a 256!  Way to small with no torque at all compared to a 312.

Many of the ’54 model 239 and 256 engines also had some manufacturing differences thus making these two engines difficult to interchange parts with from the ’55 and later 272, 292, and 312 engines.  Some of these differences include larger sized journals on the camshafts, larger camshaft bores, tanged oil pump, different design oil pump shaft, 13 teeth on the distributor gear instead of 14, and a smaller water pump with a corresponding timing cover.  Seems like the fuel pump also had a different mounting pattern.  The 312 is the obvious choice if the block checks out.

But I recently did see a 256 engine with a late model camshaft and the bearings were special.  Extra thick so that the smaller journal camshaft would fit in place which also allowed the use of the later model 14 tooth geared distributor.

For a Y in the Falcon chassis the critical point is how you want to do the exhaust because the shock towers are real close to the sides of the heads. It has been done but not by me.  Some others will know for sure but a Fairlane may be a bit wider the towers than the Falcon