Two most important things about airbags:
1. Check and double check for possible clearance problems. If it looks like it may rub or contact any metal or exhaust move it or modify the part thats close. The last thing you want is for that bag to give out while your driving it. Safety is always the most important thing consider in any modification.
2. Make sure that the airbag is mounted so that it travels properly. In other words, the upper mounting flange should always be parralell to the lower flange. If an air bag is mounted that the fanges are more than 10 degrees off, when it expands and collapses the bag can wear against itself. Again the last thing you want is for that bag to give out while your driving it.
Other tha that you can pretty much mount them any way that you want.
Be Safe and enjoy the low life.