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One way are the three freeze out plugs on the side of the motor. .....so I'm told, All small blocks have 2. The best way is to get the casting numbers off the back pad on the block (drivers side)or the Identification numbers off the front pad under the alt. then run em. Also I believe there are differences in the heads (steam holes). the motor may have to be apart to ID that however......js
all 400 have 509 cast in to both sides of the block. upside down about 4/3 of and inch high by the boss for the ft mounts. Not all have three freeze plugs though. If you look at the balancer it not unaform like the rest of the sb.
The 3 freeze plug per side identifies a 4 bolt main 400, which was only available from 1970-1972 in cars, not trucks.
The 400 was available in trucks from 1975-1980.
