Been thinking... (that's scary)
You said use origional trans. When we did this swap years ago (430 into '57), we used a big block (FE) toploader. Everything went together. I am not quite sure the earlier Y-BLOCK trans will have the same mounting flange pattern, same input shaft length, pilot bearing ID and so-on.
The Y-BLOCK trans may also not hold up to the torque of a 430. If everything matches, use the Y-BLOCK 3/S but you might want to consider finding a later 3.03 3/S (1963 up) for strength and synchronized first gear.
Don't want you cussing me on final assembly...
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