I think it is correct . We learn "what + to do " which means what is the best thing to do in Japan so, we directly translate "what to do ~" into ～何をすれば　一番良い事なのか so , we can directly translate "what to say" into "何を言えば一番良い言葉なのか" and this sentence seem to be similar to the sentence you wrote 何を言えば一番良い言葉なのかわからない。 but it is so redundant that it is not common expression though it is understandable for Japanese, but your sentence is common expression :) so you are right. so I think you are right :)

Obviously I'm not fluent but I might say something like 「何を言えばいいか分からない」. Unless you're trying to say "I don't know what I should/ought to say". Although I could be wrong ^^