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400 has lower comp & bigger dia. pistons.It also has better heads(452).906s need to be ported & hard seats installed.both have cast pistons.steel crank does not matter because you are not going for max boost with cast pistons.No detonation,this will kill cast pistons.

How much difference are the ports between the 452's and the 906's?
Well, I'll tell ya...there's not enough difference between the ports of the two head to worry about but the good part about the 452's is that they have the hardened valve seats already...not that I ever sunk a valve in 906's.