There are some things to consider. If you take the AP test this May, that doesn't leave you nearly enough time to prepare, in my opinion. Although I took AP Physics C, I don't know much about the B test. And if you spend $80 on the test, you better be sure an appropriate lower score can get you any credit at the universities you're thinking of attending.
I self-studied both AP Physics C tests (mechanics + E&M) and a couple more AP's also...and I did get 5's on them, so it's possible. It depends on how comfortable you already feel with the material, how disciplined with the self-studying you are, and how much experience you have with self-study.
Make sure you get a good physics textbook, but also, get a really good set of AP physics study guides (especially the Barron's)