run nmb cable in conduit
Stripping the sheath is prohibited

The wires you use have to be identified, i.e. have markings on them.  Interior wire in NM cable is not marked.  

You have to pig-wrestle the entire NM cable through, sheath and all.  Good luck. 

The correct wires are THWN or XHHW

If you want the flexibility of individual wires, you want THWN or XHHW type. You need a #10Cu green or bare ground and three #6Cu wires, one neutral white or gray, and two hots any other color.  They can be the same color.

If you don't like the price of #6Cu try #4Al.  

If you run #3Cu instead, you can run 100A on that wire.  You will need to bump the ground wire to #8Cu and it will need to be bare. This will be a harder pull, though.