You need to ask for a 6.2 flywheel.
Any parts store can get it.
As for 465 behind a diesel, I can say from experience, it's better than with a gas engine, except everyone is saying to go with 4.10 with your 36" tires, I say 3.73:1
I had 35" tires and 3.08 gears and I loved it.
I wanted overdrive for higher speed but I topped at 65mph.
I am now working on my crewcab with that setup and once I have it running I will report on how it does with a heavy truck and 4.10 gears.
My other setup was in a K5 light vehicle and the only problem was the 3-4 spread but as Rene said, the engine had plenty of torque to take it, you just need to get used to it.
You were saying the 5speed you have on your ranger doesn't give you trouble in mud, well it's nothing like the 465, and I have done mud with my K5, you can not shift and you don't have enough RPM's to run it in one gear.
Maybe you just keep the ranger for mud and this one will do everything else.
Now if you don't have any rockcrawling, maybe you can use the NV3500, the only problem wiht that one is no Low gear, but has a good spread on the 5 gears. It was developped around the 6.5l diesel.