Yeah, your plan to splice in a junction box in the basement is fine. The junction box will need 32 cubic inches, so I'd consider a 4-11/16" square box as a minimum. JRaef's idea of a Polaris connector is the right concept, but you'll be better off with a "mini-Polaris" such as a MAC Block.

Upsizing to #6 wire for the last 4 feet buys you nothing. You can just stay with 8 AWG aluminum.

8-2 cable is also not legal for ranges (unless it is UF cable with a ground in individual strands that spiral around the outside of the conductors. There have been a fair number of incidents of installers using 8-2+ground cable illegally. These cables have a single ground wire that is just a normal stranded wire like the conductors.

Yeah, your plan to splice in a junction box in the basement is fine. The junction box will need 32 cubic inches, so I'd consider a 4-11/16" square box as a minimum. JRaef's idea of a Polaris connector is the right concept, but you'll be better off with a "mini-Polaris" such as a MAC Block.

Upsizing to #6 wire for the last 4 feet buys you nothing. You can just stay with 8 AWG aluminum.

8-2 cable is also not legal for ranges (unless it is SEU cable with a ground in individual strands that spiral around the outside of the conductors. There have been a fair number of incidents of installers using 8-2+ground cable illegally. These cables have a single ground wire that is just a normal stranded wire like the conductors.

You can't run 40A on 8 AWG aluminum

Other than that, Mrs. Lincoln...

Yeah, your plan to splice in a junction box in the basement is fine. The junction box will need 32 cubic inches, so I'd consider a 4-11/16" square box as a minimum. JRaef's idea of a Polaris connector is the right concept, but you'll be better off with a "mini-Polaris" such as a MAC Block.

Upsizing to #6 wire for the last 4 feet buys you nothing. #6 aluminum is indeed 40A, but you can't just go "la la la, the wire I can see is #6 therefore I can run 40A"... no, your max on that circuit is 30A because of the 8 AWG cable present.

That also means you cannot install a range which requires a 40A supply. Separate cooktops or ovens that are 30A are readily available; so maybe an electric oven and gas cooktop for the best of both worlds.

8-2 cable is also not legal for ranges unless it is UF cable with a ground in individual strands that wrap around the other conductors.

Yeah, your plan to splice in a junction box in the basement is fine. The junction box will need 32 cubic inches, so I'd consider a 4-11/16" square box as a minimum. JRaef's idea of a Polaris connector is the right concept, but you'll be better off with a "mini-Polaris" such as a MAC Block.

Upsizing to #6 wire for the last 4 feet buys you nothing. You can just stay with 8 AWG aluminum.

8-2 cable is also not legal for ranges (unless it is UF cable with a ground in individual strands that spiral around the outside of the conductors. There have been a fair number of incidents of installers using 8-2+ground cable illegally. These cables have a single ground wire that is just a normal stranded wire like the conductors.