Correct me if I'm wrong? Throught the years I heard it through the grapevine that the 340 fitted with cast cranks were also fitted with a different trans package. And if you wanted it to have the full value of balancing technology that package offered you could not swap the trans with one other unless it was of another 340 cast package without compromise!
340 automatics were all 727's from the factory. The only differences were the single wire neutral switch type in '68 and the externally balanced converters in late '72-3 to match the cast crank engines after April. 1, 1972.