Looking into this a bit more (Google finds an amazing amount of information), it would appear that your '98 engine may have screw in studs with stamped steel rockers, just ones with a shoulder on them for the net lash valvetrain setup used from the factory. It wasn't until 1999 or 2000 where the rockers were changed to bolt down with the use of a rocker arm support.
Summit Racing has THESE ARP STUDS
that state they are for Vortec 4.3L engines. Maybe you can call ARP to confirm? Or Summit? Cam manufacturers will also likely have answers to this very question. Just because they only sell the cam doesn't mean they don't know specifics about the engine that it goes into and what other supporting mods are required to run said cam. They wouldn't be very good at manufacturing cams if they didn't know the limitations of each application and how to make the appropriate changes, if required.
One thing to watch out for is valve cover clearance. Changing rocker arms could cause an interference between the rocker arm and the valve cover. I'm not sure what your options may be for aftermarket valve covers that would add clearance in this area. In the past, where aftermarket alternatives were lacking, I have seen people machine spacers out of aluminum to move the valve cover up and out of the way of the rocker arm. That's $$$ though.
Having no clear direction and asking specific questions to gain direction is one thing. Asking vague questions and relying on others to do the research for you is another. Google is your friend, use it. If you find conflicting information, ask away and we'll see what we can do to get you the facts. Keep on the path you're on and you'll find yourself complaining even more about the lack of responses. I'm trying to help you get responses. Show effort and it may result in more people answering up.