Interesting discussion re “spelt” vs “spelled” at English Language & Usage Stack Exchange, which, incidentally, is a question and answer site for linguists, etymologists, and serious English language enthusiasts. It starts with the question:
In the following sentence, should I say spelled or spelt:
You spelt/spelled “Pneumonoultramicroscopicsilicovolcanoconiosis” [!!!] wrong.
See below for apparently authoritative conclusions from grammarist.com.