The 6.5 is a one-piece rear main. The biggest headache is getting to it (remove tranny, flexplate) but then its just prying out the old one and putting the new one in.
Its a bit tricky getting the old one out. I like to take a sharp knife and cut the rubber out of it so I can get some space to pry. I like using an aluminum or brass drift to punch the old one out, that way I don't risk scratching the crank or the bore.
Installation can be done by carefully tapping around the perimeter, but you risk hitting the crank or deforming the seal. I suggest finding a piece of PVC or steel pipe that is the right diameter and using it as a big round seal driver.
Coat the crank's sealing surface with oil so you don't damage the seal, and you can coat the outside of the seal with oil too. Some guys use silicone, but I've never had one leak on the outside of the seal where it presses in.