when i had a van i had one of these:
http://www.thule.com/Thule/ProductPage____33623.aspx
it was the business for transporting more than one bike
But, when only transporting one bike, it was unnecessary to use it, as my bike would stay reasonably still just by having it lying on the floor with the handlebar in between the raised ply lining and the side door.
however , if you're regularly transporting multiple bikes i'd recommend it. a transit's got about 8 foot load length? that only needs about 5 feet (length of bike minus a bit) so you'll have room for other stuff aswell, or you may even be able to get it sideways.
Consider just bolting it to the floor, that's what i did (DO check to make sure you aren't drilling into the fuel tank
)