Anytime the loaded trailer weighs as much or more more than the tow rig, you're getting into potentially dangerous territory. a 350 engine and a SM 420 transmission, or Maybe a Turbo 400 automatic will have enough oomph to get the load rolling, assuming you've got a 3.50:1 or lower geared rear axle. A 3.73, 3.90, or 4.10 would be better if you plan to do a lot of towing in hilly terrain. A load leveling hitch and very good trailer brakes will be a must. Most boat trailers use surge brakes, which don't impress me much. I prefer to be able to apply trailer brakes only to get a sway under control, which can't be done with hydraulic brakes and a surge type coupler. LT rated tires are going to be a necessity as well.
As Ron has already mentioned, individual driving habits and lots of experience towing a heavy load will compensate for marginal equipment to some extent, but pushing the envelope with either equipment capability or running faster than is wise can get dangerous in a big hurry. Set up your tow rig as thoroughly as possible, and drive carefully. Have fun and be safe!
Jerry
