Could it be that the "CONV 4 1286" might actually be an "I"? That would work out to an Oct. 28, 1956 date, which would be a '57 model year engine. As far as dufferences, I think that the passenger engine had a slightly higher compresion ratio, and a different cam grind (most likely hydraulic). Other than that, they're pretty much the same. As far as to why it got changed. I'll bet the original engine got lunched, and they just used whatever they could find to replace it with the least amout of $$$ and work involved. This wouldn't be so noticable in a Chevy truck, as the engines are so similar, but a GMC used a completely different engine.
