391 is for manual steering & 390 is for power. Book says all with p/s. Physical difference, unknown. I've heard more plies for power. I'm going to check the splines this morning....Joe
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1968 Z/28 Corvette Bronze. Black Hounds Tooth. 02E Los Angeles born 3/13/1968 pnt OO
1969 SS396 Yellow/Yellow 08E Norwood born 8/28/1969 pnt 76E
1970 'cuda Moulin Rouge, 440-6, 4 speed
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