One should never run straight antifreeze. First off, it doesn't cool as well as a 50/50 mix. Secondly, at bitterly cold temps, it will create a slush in the system that impedes flow.
Half-and-half mix tests to -40.
As far as completly draing blocks: Walley-World, or your favorite department store, as well as auto parts and farm supply stores carry testers. Change your antifreeze, but don't completely fill the rad or water bottle. Drive for a day to get it mixed really well, then test. Add straight antifreeze, or straight water, dependant on whether it's too strong or too weak, until the system is completely full.
You're good to go!
Tn