According to "How to Build Max-Performance Mopar Big Blocks" this is largely a myth. True, Mopar Performance did publish that 76-78 block had thin walls and should not be bored more than 0.030" over but sonic testing has shown that late model blocks actually tend to be thicker than earlier blocks. The book goes on to state that quality control may have decreased during these years and that the blocks may subsequently be more prone to core shift.
Either way, I would feel more comfortable putting my hard-earned money on a pre-'76 block. You should have it sonic checked regardless though.