Actually a trucks payload is determined by the trucks GAWRs and in particular the trucks 6084 RAWR as its gonna' carry all the load. GVWR doesn't determine how much load a truck can carry from a legal perspective. You can use the GVWR or RAWR payloads if you choose. Both are safe.
RAWR payload example; the '98 2500 with the four spring pack main plus the over load spring pack is a haulers dream for that size truck.
You didn't give us any front or rear axle weights but many may weigh in the 2700-2800 lbs range which leaves around 3200-3300 lbs for a payload. Your choice but which ever route you go a truck should always have front and rear axles weights. You sure don't want to go over OEM rear suspension/wheels/tire capacities.