Is one form wrong or more correct than the other? Or do they have different meanings?
I'm a non-native speaker trying to figure it out.
The difference between finally was and was finally is the adverb/verb order. Here, your verb is "was", a form of to be and the adverb is finally. There is a good guide on adverb placement, and some listed rules are:
Because your sentence uses was, a past tense form of to be, the second rule applies: The proper word order is:
Note that if you do happen to invert the adverb/verb order, you will be understood in most cases but it won't sound right to a native ear.