Home Alone face Gosh, I had not realized I've been doing work wrong all these years!The copper wire I purchased is #3 gauge and matches the original service. I bought a Polaris connector and returned it. The Polaris connector does not seal out air to prevent oxidation.
Listen. Your attention to detail is important. It is your best defense against bad work. But also pay attention to experienced people and best practices. Also, I think you're attentive enough that it's time to start shopping at real electrical supply houses, so you can talk to clerks who actually know stuff. You're beyond Home Depot stockboys at this point.
Also, buy the wire last
Polaris connectors are correct for #4 or larger wire, unless you really, really know how to do a pro job of dual-layer taping, and want to spend 10 minutes per splice doing that. Get Polaris connectors whose *maximum* size is your larger size, that way the large wire fits snug.
If your wire is #6 or smaller, I just discovered MAC Block connectors, and they look excellent.
And by the way, we solve the oxidation problem by using the correct anti-oxidant "goop". Oh, and this will pop your cork: The Alumiconn, MAC and Polaris connectors are made of aluminum. So yeah. All your assumptions about dissimilar metals... And this aluminum wiring rabbit hole goes a lot deeper and has a surprise ending, but I won't ruin the suspense
So, just as jbfan points out, you have an obsolete range/oven connection (and #3 aluminum wire, what on earth were they thinking? How did they even get the wires to fit on the breaker?) You are not allowed to do anything with this obsolete connection. What you really need is a new home-run for the new appliance.
Retrofitting ground. Well, we may be able to salvage it, if the neutral wire is white. If it's bare, the cable was the wrong stuff *from day one* and cannot serve anything but a 240V-*only* range (i.e. one that takes H-H-G and does not want H-H-N-G). If it's a mesh wrapped around the other conductors, that's legal to use as a neutral, but it's risky to retrofit ground to, because you must prevent contact with bare mesh neutral and ground.
Since the circuit is #3 aluminum you must use a #8 copper ground wire (when you bump the conductors you must bump the ground). It must make a homerun either back to the panel, back to the Grounding Electrode System (the copper wires going to ground rods or water pipe), or to the nearest steel electrical conduit that is continuous back to the panel.
However, the electrical inspector won't approve doing a retrofit ground and then immediately extending to a new location. They'll expect you to home-run to the new location from the start with appropriate cable. You could retrofit ground *and then 24 hours later* extend to the new location, but the inspector might not buy it.