Improving the accuracy of AK-47 has everything to do with manufacturing tolerances, longer barrel wouldn't nessesarily help not if using the original 7.62x39mm round, it has so loopy trajectory that longer barrel would not be so beneficial, that's my opinion.
However if a caliber such as 5.56mm nato is used the longer barrel would indeed be factor in improving the accuracy, still those tolerances would have to tighter and material quality good.
I dunno about 7.62x51, how's that for AK's, I've seen couple of Valmet M76's chambered for that but that's all.