How to Shorten Semi Trailer...

I would worry more about the horses going through the sides than the floor. What about lowering suspension, and low pro tires and wheels. Possibly 17.5s. Don't know what would be feasible when it comes to that but I know a lot of trucks and trailers sit much lower than mine.

 

I was looking into having this done on a straight truck. I run a lot of loads in NYC and some docks are lower than 12'. My truck is 12'6". There are truck body repair companies that will shorten the height of your trailer. I got quoted around $2000 to shorten height on a 26' box. It is a lot of work to drill all those rivets out, redrill new holes after cut, rivet back together and reseal. Sometimes its easier and cheaper to find another shorter box. I'm leaving mine the way it is, and installing a dump valve in the suspension. My next truck will have a shorter box.

 

Ive got a 53 ft 14 horse slant trailer with a 16 inch drop in it. entire rear load folds down just like you are wanting. Id say the trailer I have has close to 10k in the rear ramp and hydraulics alone. The ramp is aluminum as well probably weighs close to 2000 lbs. I know i cant even begin to budge it with the pressure released. Not sure what the budget for your build is but you would probably be better off having a trailer spect out like you want it bare and then put the living quarters in yourself. The inside of my trailer has kick panels 5 ft up. original white thin trailer panel and then 1/16 inch steel behind that from the floor to the ceiling, insulation and supports inside, then the outer shell of the trailer, then covered in stainless steel. Even with all that a horse can move the outer wall if it throws a fit. To buy the steel just to hold the sides up enough for livestock would cost a fortune. Also with a folding rear the horses go in fine but getting a vehicle in there might be a challenge because of the height the trailer is off the ground. You will need some good ground clearance. You might check with turnkey trailers and custom coaches they built the ramp on mine and the rest of the conversion

 

I don't know why everyone finds it so impossible to modify anything a human built it so a human can modify it. A drop deck furniture van will have a fairly substantial frame under it , so if you want to lower the roof a foot or so it would be a lot of labour but not really hard, if you have some decent fabrication skills. the roof should be fastened to the walls with a row of rivets or huck bolts, these would need to be drilled out, the roof structure supported on scaffolding and then make your cut , lower the roof evenly and start drilling and riveting, when you done you have something to be proud of instead of a cooky cutter with mess of payments.