What happened to the 318 in 1971?

Yeah me too.
But the trend is clearly visible; beginning in 69 the deck clearance was increased a couple of times and, beginning in 71 they messed with thicker gaskets.
During a rebuild this all can be changed. Any teener can be used as a base; the forged crank engines being possibly, more desirable.And the large-rods with floating pins,too.