Er... The GM 151cid/2.5L (Pontiac's "Iron Duke") used hydraulic lifters. There should be no valve adjustment. If you're getting excessive valvetrain noise, you probably have a collapsed lifter or blocked oil channel somewhere.
On the off (and somewhat improbable) chance that you have mechanical lifters/solid camshaft, Crane lists three lash adjustments for solid and roller camshafts in the Iron Duke:
Mechanical Lifters, 112° LSA Cam:
.028" Intake / .030" Exhaust
Mechanical Lifters, 106° LSA Cam:
.020" Intake / .020" Exhaust
.035" Intake / .030" Exhaust