Not sure I understand what you're saying here...
120-80 is not a stock size tire for the front of a C10...'94 and newer came with a 120/70...
Are you saying you lowered the tire pressure to 30 lbs...?
I am running a 120/70 at 38 lbs pressure...and haven't found any unhappiness with it...
Not sure there is a 120/80 that would find its way onto the front of my 2003...but, I may go to a 110/80 next time...
That is... If I don't need to find a 17" front...