When Penning was drafted and they went over his strengths. Mauler needs work as a pass blocker. I was screaming RT or OG because usually your weaker pass blocker like Zach play the right side or guard where you can cover up their pass blocking.
He may be better inside where he does not have the edge going against him as he's never really displayed the feet to play tackle but as you stated being a mauler is what you look for in an inside guy and he has great size for a guard.