The '99 8600 GVWR package is the old C6P chassis with a 4L80 tranny with the full floater 10.5" corporate rear axle. This is one very tough truck for its era. The '99 may have been the last year for the C6P chassis and the 5.7 engine.
IMO the 7500 lb tow rating sounds a bit low as my older '90 2500 LD 5.7 throttle body chevy 7200 GVWR came with a 8000 lb tow rating. I towed doubles (9400-9500 lbs) for over 8 years with the truck. Even towed both of my GN stock and a flatbed trailers with the truck.
The '99 5.7 CPI engine has much more HP/torque. Something like 250 hp/340 torque sounds familiar. I would stay under 8000-9000 lbs tow. The '99 4x4 may weigh 2600-2700 on the rear axles which leaves around 3200-3300 lbs for a max axle payload.
