The 76" came from the TM. I have never owned an M-101A1 or been able to compare my A2 to the A1. So I really don't know for sure. I'm just trusting the TM.
I've been wanting to turn my trailer in to a camper as well. I looked at the s250 shelter but I just felt it was too small. I wanted something bigger so I can store all my camping gear in it too.
I also considered putting an M-105 box on a M101 trailer body too but I found the modifications to be more trouble than it was worth. If you're going to go that route, you're better off just getting an M105. The best thing about the M105 is that they are cheap and easy to find. There are only two draw backs. One is that it's about a 1,000 lb heavier that the M101 (2,650 lb). If your vehicle can tow that, then no problems. The other issue are the brakes. You will need to replace the air brakes with an electric or surge brake. After some considerable thought, I think I'm going to sell my M101A2 and buy a used M1101 or 02. (I've seen them around the $3,000 range). The cargo bed is actually wider than the M105 (M105 - 74 / M1101 - 80.3) The bed length is slightly longer than the M-101 but about 2-1/2 feet shorter than the M-105. The big bonus is even though it has a much bigger cargo bed than the M101 is still weighs about the same at 1,460 lb. And since it was design for the HMMWV, which I have, no modification is required.
On the brakes. I would definitely get one with surge brakes. There have been a couple of time where I had idiots cut in front of me towing a 3,000lb load and it was a wonderful feeling to feel the surge brakes kick in when I had to hit my breaks. The other nice thing about surge brakes is that it only cost me a $150 to modify it so that it could be towed by a civilian vehicle. All I had to buy was a combo ball & Pintel hitch for $115 and $35 for add on 12v trailer brakes lights that you simply screw on the back of the tail lights. It took only 10 min. to install. So now I can tow it with my HMMWV or SUV. If you had electric brakes you would then have to deal with the 12 vs 24 volts.
Two other things I would invest in is an adjustable landing foot for the back and a hand crank landing foot with a wheel in the front. This way you can use both the front and back legs to level the trailer as well as keep it from rocking. Also having a landing foot with a wheel makes it much easier to move around by hand and hooking it up to the vehicle by yourself.