It is not a wise idea.
225 50 16= overall diameter of 24.86
225 70 16= overall diameter of 28.4
The result will be a higher center of gravity and more sidewall. That means your handling will suck and not to mention your speedo will be off by 10mph.
Use a tire calculator like this one to compare sizes:
http://www.discounttire.com/dtcs/infoTireMath.dos