Mooni : I believe you have to tow in THIRD gear and not Drive or Overdrive with that year transmission. There may be something on the door frame and ownersw manual about that.
The OD gear on those older trannies can not take the load, they will burn up. Third is the direct 1:1 gear through that transmission.

Wannabe

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.

hey man do not spend lots of money doing some of this dumb S***, if you boat weighs under 6500 tow it. Knock the thing down into drive and haul away. If you feel as if its struggling go into third. We towed travel trailers and boats for years with the chevy 350 and worked fine no tranny or engine failures. It amazes me at times that people think 10-15 years ago was a century ago and forgot how things went down