I've transported acetylene bottles on their side in my pickup truck bed. It's not illegal or dangerous. However, the bottle can't be used immediately after being on it's side. The bottle must sit upright for ~ 1 hour so the acetone can drip back through the sintered metal and leave a gas pocket for the acetylene.
Also, the size of the bottle determines the rate at which acetylene can be removed - it's a fraction of the bottle size. The ratio of acetylene to oxygen is 2:5 or ~ 1:3 , but the oxygen bottle does not have a flow limit like acetylene so the size of the acetylene bottle will determine the maximum welding thickness. For 1/4" steel, one of the larger personally owned cylinders should be big enough the last time I checked.
Most (important) things to know about acetylene and oxygen welding and cutting are here: http://edgewh.esabna.com/EUWeb/OXY_h.../589oxy1_1.htm