I currently have 5 different units sitting waiting on engines, everyone torn apart with photos for warranty . Pistons gone, holes in the block, most with under 5K. If you're a tech and on facebook join the Ford tech page, answer the questions and you get to read the horror stories. Focus...
I have personal experience here, I took apart a 93 Destiny a few years ago. I'll be brutally honest, chances are that there will be so many rusted and rotted parts that it wont come apart any where close to how it was put together and you may end up with a frame only
Putting bags on a 450 towing a 5er is a whole lot different than towing a bumper pull. Bags on a bumper pull add to the fulcrum, thus transferring the weight off the front wheels. A 5ers tongue weight is dead on top of the springs, no fulcrum effect.
In 35 years as a dealer tech I've never seen any warranty voided due to towing. You first stated it was #3500, then you said under #3500, if you're that close, get one. These will squat quite a bit.