After torquing the head bolts to 33ft lbs on 1995 Chevy 31 how do you measure for 90 degree turn they call for?

There is no 90 degree turn called for in the 1995 Chevy 3500. After the initial torque of 33 lbs ft, you then retorque the headbolts in the sequence described in your manual to a final torque spec of 115 ft lbs.