These engines use hydraulic lifters so there really isn't any adjustment to be done on the valvetrain, all you can do is check the rocker arm bolts for tightness (do not over tighten!) or replace faulty lifters.
Here's the functional description of the valvetrain from the service manual:
Motion is transmitted from the camshaft through the hydraulic roller valve lifters and tubular pushrods to the roller type rocker arms. The nylon valve lifter guides position and retain the valve lifters. The valve rocker arms for each bank of cylinders are mounted on pedestals (pivot supports). Each rocker arm is retained on the pivot support and cylinder head by a bolt. Valve lash is net build.
"net build" is gm-speak for you just bolt it together and it works without any adjustment (other than the correct torque on the bolts).
The service manual does not have a valve adjustment procedure, however this is what it says about installing the rocker arms:
Rotate the crankshaft until number one piston is at top dead center of compression stroke. In this position, cylinder number one rocker arms will be off lobe lift, and the crankshaft sprocket key will be at the 1:30 position. If viewing from the rear of the engine, the additional crankshaft pilot hole, non-threaded, will be in the 10:30 position. The engine firing order is 1, 8, 7, 2, 6, 5, 4, 3. Cylinders 1, 3, 5 and 7 are left bank. Cylinders 2, 4, 6, and 8 are right bank.
With the engine in the number one firing position, tighten the following valve rocker arm bolts: Tighten
Tighten exhaust valve rocker arm bolts 1, 2, 7, and 8 to 30 N·m (22 lb ft).
Tighten intake valve rocker arm bolts 1, 3, 4, and 5 to 30 N·m (22 lb ft).
Rotate the crankshaft 360 degrees.
Tighten the following valve rocker arm bolts: Tighten
Tighten exhaust valve rocker arm bolts 3, 4, 5, and 6 to 30 N·m (22 lb ft).
Tighten intake valve rocker arm bolts 2, 6, 7, and 8 to 30 N·m (22 lb ft).
Before you do all that, have you changed the oil and filter? Using the wrong viscosity oil or having dirty oil and a dirty filter could give you some trouble.
