As 66fs mentioned, it was the change made from a '67 273 forged crank to a '68 cast one. The transmission made the same change internally that year....across the board to all 904's and 727's both small block and big block versions. The slant 6 engine also made the same crank register diameter change the same year. All 318, 340, and 360's had the same large crank register. Manual transmission cranks received a final machining for a manual trans pilot bushing, but, most automatic applications didn't get that machine work.