Different motor mounts, Different tranny if auto or different bell housing if manual. The correct 318 for 66 and earlier is the old style 318. Not sure if it uses different mounts then the newer style 318.
poly As will bolt up to LA pattern and vice-versa, the dowels and all but one lower bell bolt are in same place, the 7\16s bolt on the drivers side( I think) is about an inch off, won't hurt a thing, you still have six bolts holding it on. Pre 62 poly has longer crank flange where flex plate bolts on. since you have to change bells or tranny anyway, get one to match intended motor.The radiator has different outlet locations from v-8 also. I'm thinking the old style p wagons (pre72) had the oil sump at the very front of the pan. I believe post 61 poly shared the motor mount bracket ear pattern with LA.
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