can the cwp of a 7.5 diff be transferred into a 7" casing . I have literally spent hours reading various answers and now nithing makes sense.

I dont think it can from memory, but there are others here far more qualified to answer!

When I bought an ATB i had to specify 7 or 7.5 which is why I dont think the crown wheels are interchangable

I am almost certain the answer is no. If it could why would Ford have bothered with a different housing for the large diff?