I would concur with above; get the letter now and if need be, you can ask for an update later on.
You can take the MCAT anytime prior to your Application. If you are planning to apply June after graduation, you could take it that spring (April or May). However, that might give you lots of studying/prepping while trying to enjoy the last of your college days. You may want to consider taking it at the beginning of your senior year in September) . You could also prep that fall of senior year, final push during winter break and take it just before or early in your Senior year