Joining #3 gauge aluminum to copper wires

My new range/oven requires a 240V-50 amp "Y" plug outlet to be mounted on the stud-wall base. The existing plug is mounted about 6 inches too high My solution is to mount a new outlet on the stud-wall base and join a 3 gauge extension from the original junction box to the new outlet The solution works, although this creates a problem

After I opened the wall, it appears that the original service is 3 gauge aluminum. To have a working range/oven, I joined the two wires using wire nuts. I cannot find an aluminconn wire nut or some other solution intended for joining dissimilar metal wires of this gauge.

Is there an easier solution besides replacing the original service?