Yes, you can use a later 3 speed in your 3600. You'll need to modify the shift arms a little depending in the transmission you choose, and you'll also have to replace the the driveshaft to the carrier bearing with an open one as you can no longer use the torque tube.
"Something still is making sence to me about this post. Is this a 3600 or a 3800?"
If he's got a stock 3 speed in it, it's a 3600.
