You cannot adjust the standard rocker arms. The cam followers are hydraulic & do this automatically.
You can adjust the height of the pillars (pedestals) that the rocker shafts are in.
You need to take measurements & then work out the thickness of shims required.
One shim goes under each pedestal & the shims must all be of the same thickness for each shaft.
Read 'checking tappet preload' in this link:
http://www.v8engines.com/faq-gen.htm#preload
Alternatively you can buy pushrods that are adjustable. But you're really just overcomplicating things.
I used Elring composite head gaskets when I overhauled the top end on my 3.5EFi. Very light skim on the heads, less than 10 thou IIRC & no adjustment was required.