Well, the '89 wouldn't have magnum heads from the factory. Now maybe somebody put a magnum in the '89 truck. Magnum heads oil different and don't have the rocker shaft. I'd verify they are magnum heads before doing anything.
Good to know. They do have square rocker stands. No rockers just valves & springs. I only have what he said and he did say "magnum". He said the truck was an "89", nothing about the motor.