Your statement about parker frame sizes is no completely correct. All the frame sizes from 000 up to 7 are all different in pretty mich every way. Width at hinge and stock, firing pin spacing, bolster sizes and height of standing breech.
The only frame sizes that share the same frame width is the 1-1/2 and 2.

And the cause of cracking on most all ithaca doubles is the stock loosening up on the action. Drawbolt or not.

B.Dudley