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You should be using Google Content Experiments. It used to be called Google Conversion Optimizer. You can find Google Experiments in GA > Content > Experiments. There is a video there you can watch. You can test up to 6 version of a single webpage.

Here is SEOmofo's article on the (not so) recent change. It is safer to limit your characters to 70 rather than 77 as some titles that are 77 characters are more likely to get truncated in the SERPs. The SEOmofo snippet tester is good because it factors in pixel width rather than character length.