"The 383 was a bored out 361"?

[YY1]

I think they wanted to switch the 383 to the 4.32 bore of the 440. But that resulted in a 396. So they increased it to 4.34 making the 400.

I think the BB development theory stated earlier in the thread was correct-

400/451/470

...and "ball stud" Hemi in various displacements.