There is a lot of work to swap in a six cylinder where a four cylinder was. First, the transmission has to be swapped out with bell housing and clutch. The transfer case has to be modified with its input shaft to match up to the new transmission. Then the driveshafts have to be modified. Your shifting linkage needs to be modified also. There are more things but these are the basics.
If you are interested, check out my signature below and look at my rebuild. It goes into a V6 swap I did, but it is pretty much the same you will have to do... except for some specifics for your engine.