Splicing 8 gauge NM cable?

I need to move a double pole, 60 A breaker in my panel (controls the heat elements in my furnace) to another location inside the same panel to install an interlock. Running a new length of cable is a no-go as it terminates in the nether region of the furnace that is not accessible. The cable has a black jacket, looks to be 8ga, three conductor plus ground (I haven't poked my head up there to read the jacket yet).

How do I go about doing this? Is it as simple as adding another 2-3 feet of cable at the panel and connecting the ends with some wire nuts inside an accessible junction box? If so, any tips?

Thanks.

 

It should be 6 awg copper and yes using the proper size wire nut in the proper enclosure will be fine for extending the circuit