I've been in 3 shops over the years with dynos: 1 roller/chassis dyno (which actually used a water brake, not a mass roller. It was old when we installed in in 1994) and 2 engine dynos(1 water brake, 1 eddy). All of them were part of rhe main building. Both engine dynos were outside walls, with a concrete filled cinder block wall and bullet resistant glass separating it from the shop. The chassis dyno was just near one of the roll up doors, but we kept the cars facing outward, so the engines were outside the shop. We never scattered an engine with it, but the sheet metal building wouldn't have done much for safety. For practicality sake,.I would have the dyno room at least attached to a wall of rhe shop, with "internal" door access. The cost of a needed second set of tools, or lost time running back and forth to get tools, would irritate the crap outta me. Also, I'd think running power/etc would be easier if it were attached. As for the lifts, I'd put either 2 or three angling in along one wall. (Depending on space). You could have an aisle wide enough down the center for access, but by having them angled, like a 1 direction parking lot, getting the vehicles in/out and on/off would be easier. I'd also consider putting in another door between the 2 sides, so that if you need to fetch something from one side, you dont have to go all the way to rhe front, and around. Im a bit lazy, but inalso have some foot issues, and minimizing the amount of walking seems like a good idea.