you really need the year to, and the casting #'s on the back right side of the block. the YB was used for several motors
but being a 77 you really hope for a W72 400 with the Y6 or WA code .
from 77 - 79 the W72 400 was the icing on the cake. other then finding a SE
really look at your block code, make sure it's not a Y6 , some times it will look like YG
if its a 455, 455 will be cast into the left side of the block, beside the fuel pump
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