Ive wondered how much our trucks can really tow as well. Factory rates our v6 towing at 3500# if the trailer has brakes. 1000# if it does not. Our biggest obstacle is having under 150 hp and marginal brakes i think. The frames look pretty solid and have a bit of bracing. I haven't really tested the rear suspension, but enough tongue load would really compress the lighter duty suspension. One can add leafs, air shocks etc. though.

Im currently tackling the Motor and brake issue. Mitsubishi built larger motors than the 3.0 and also put better brakes under the later monteros. In the next couple of weeks I will be swapping over the disc brakes on my 95 SR to see how much more of an upgrade that is. At the least the front calipers are thicker and we go from single piston calipers to dual. Rear discs will be a huge benefit as well.

For the motor I'm hoping to have a solution within a couple months.

How heavy is your truck and trailer?