Just to make sure it was clear, I am not saying your heads cannot use factory offset rocker arms, I am only saying you will need to mock them up and check if stock offset will work. If they have plenty of pushrod clearance with stock offset there would be no need to use a greater offset.
What valve diameter is in your cast iron heads, for the valves you are hoping to use? The factory cast iron head intake valve is a 2.08" head diameter, the exhaust is usually 1.74" diameter, and I think all of the stage VI heads came with the seats cut for 2.14" intake and 1.81" exhaust. That is why ask about the valves you have, since that is what you are hoping to use. Nothing at all wrong with trying to use parts you have, if they will work.
