Ok iv'e gotten this far and would like some advice. This is a range junction box/outlet install using 6-3 wire, 50 amp breaker. Looking for tips on the ground connection to metal box, how much wire should I strip back.

Thanks
Patrick

 

First off you need more wire in the box. I would suggest at least 6-8" of free conductor hanging out. The outside jacket should be striped back so there is at least 1/4" inside the box.

Second, you just need to connect the bare ground wire to the metal box using either a grounding pigtail, or just a green ground screw. If you use the screw method, loop the ground wire around the screw and leave the end free to connect to the range receptacle.
Examples:
Ground screw: Ideal Combo Grounding Screw (10-Card)-774042 - The Home Depot
Ground pigtail: Ideal 12 AWG Solid Pigtail with Screw - Green (50-Pack)-30-3392R - The Home Depot

I also suggest installing a 2 gang mudring for ease of wiring and installation of the receptacle.

Screw goes in the green circled hole:

 

Is that box 1 1/2" deep or 2 1/8"? It should be a deep box with a 5/8" 2 gang plaster ring.

 

I really hope you have extra cable to slide into the box. The code requires at least 6" of free conductor in the box.

 

You should use the deepest box you can find. Those range receptacles are deep and the wires are hard to bend.