Larry,
I had a 1974 400 #481988 that the main web was cast with a notch,to clear the dipstick tube. It appeared to be one year only. None of the 67-78 blocks that i have seen had this.
Does your 74 350 have it, and if it does,do you have a photo?
If you were to drill for 4 bolt,the bolt hole would be thin on the dipstick tube side.
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