Chris,
My smoker is electric now, so propane should be OK if you can hold a steady 225f-107c.
Weigh your gas bottle, fire up the smoker to 225f for a couple of hours then weigh again, that'll tell how long your get from full bottle.
Your need another shelf between the wood tray and the pork, this is to hold a large S/steel or enamel pan of water or sand, this helps stabilise the temps and catch the drippings. If your using sand cover it with foil. Water will give you a slightly moister product but will need more attendance.
This is a diagram of my smoker
http://www.virtualweberbullet.com/partschematic.html you can see the relationship between the heat source (charcoal grate) and the water pan.
This is the method I use
http://www.virtualweberbullet.com/pork1.html, one tip don't be tempted to keep opening the door to have a look, if you have to open the door say for spraying apple juice then top up the water pan at the same time, before you open the door raise the temps by 20f for a few minutes before and after.
LD good luck don't forget the pics