Maybe Mike B can come along and confirm or correct what I have suggested.
I think you nailed it Dan!
Here's what I can add...
The block casting number 8378307 span 1958-62.
The 235 engine code suffix "J" was used from 1957-62
I see the block date code based on the pictures breezy supplied after you posted as I 3 7 (September 3, 1957) which like you mentioned above, more than likely had it end up in a 1958 model year truck.
Mike B
