Well I would say it depends on which 351 but none of them will work they are all counter weighted and the 4.9 is internally balanced if you mix the flywheels the motors will shake them selves apart.
-Johnboy
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-1979 F-250 4X4 400 4spd
-1989 Bronco 300six swapped in ZF 5spd
-1967 Mercury Comet Sports Coupe