Which is more correct, "it fit really well" or "it fitted really well"? The short form is something I encountered when moving to the US, I never heard it in the UK.
The definition of 'fit' used here is given by the OED (5.a.) as:
One of the two earliest quotes for this definition in the past tense uses 'fit'; the other uses 'fitted'.
So both forms have been in use for a long time.
Searching COCA, COHA, and the BNC for "fit well" vs. "fitted well," I found "fit well" outnumbering "fitted well" by 20:1 in COCA (indicating American usage), 8:3 in the COHA (indicating older American usage), and by 2:1 in the BNC (for you Brits). So either is fine, on both sides of the Atlantic, but "fit well" is much more common in modern American usage.
I think there are two separate verbs here that are the same in the present tense but different in the past.