The '99s are "old" style with the Vortec 350/4L60E and leaf springs in the rear. The same 4L60E that's rated for 12k gross. If the trans is hunting for gears or going in/out of torque converter lockup, you'll burn it up.
Your best bet is to find a kit that will let you manually lock the torque converter. That way if you have to tow without the overdrive, you can still lock the converter. Running in 3rd/"D" whatever, without locking the converter, will overheat the trans real quick.