is this a one-time thing or will you be doing this on a regular basis? either way someone will likely be along to tell you towing 1400-lbs overweight is ok. i'm not that guy. can you pull the truck? sure. but can you stop as quickly as you could if you were towing nothing or a 3500-lb truck? nope. the brakes on your MH are engineered to stop the GVWR of your MH. even with an aux braking system your braking distance will be dramatically lengthened. you're going to stress the suspension, hitch, possibly the tow bar itself if it's not rated for the weight of your truck. weight ratings exist for a reason. you can probably get away with it once, maybe more, but IMO it's like playing with matches.
all that being said i do have a good friend who towed a park model from northern illinois to northern wisconsin with a 6-cyl pickup. this was 30-odd years ago so i don't recall the specifics but i recall him telling me that it was quite the experience and he was glad it was a one-time thing. we were all young and dumb at one time. most of us survived. my unsolicited advice is the same i gave my good friend...please think twice about doing this.
good luck.