Jeeze Fred - you beat me to it!
I have towed a 12,000 lb trailer quite a few times with a 3/4 ton truck that weighs about 6000 lbs empty. I have a high level of respect for the loaded trailer and what it can do to you going down hills and around corners and in heavy traffic. Your power estimates are fine. You should plan on a 4L80 transmission if you want an automatic and you will need an electronic box with it to make it shift right and a 4.11 14 bolt rearend with 16" tires will put 60mph right around 2100 rpm.
Forget using the original rearend and whimpy brakes. . . unless you have the flowers all picked out for your casket.
Getting 8-9 tons rolling down the road with a 3/4 ton truck is doable - but careful attention to suspension, drivetrain and brakes are a must if you want to do it safely and reliably.
