Sirsureshot/TT91...its because many people feel that the 331 is more reliable and safe under boost than the 347. some 347s have also had oil problems due to the longer stroke. On top of that the fact that the cylinder walls on a 347 are thinner so it could make the block weaker. There has been many threads on this site debate the whole 331 vs 347 topic but there is no right or wrong answer