I have set up both my BK and MT/LT as bottom dump keggles, but I cut the bottom out of them, took all the guts out, and attached a 2"x1/2" triclover to NPT fitting. The clover fits perfectly to the sanke fitting's lip. If you already cut the top off, you won't be able to go that route, but you could look around to trade your cut keg for a whole one.
If you don't want to go with the tri-clover you have some options. Placing the drain in the center will allow you to drain the most liquid, especially if you solder or weld the fitting in flush with the bottom of the keg. Putting it off center will leave some wort in it, but will allow you to whirlpool. If you do go with the center drain I would recommend installing a false bottom to strain the wort (if you use pellet hops you'll need something even finer, like a hop blocker).