I bought a new electric kitchen range to replace my old one. The installation instructions say that my hard wire connection needs to use 8 gauge wire. I have 10 gauge wire coming from my junction box and a 40amp breaker in my electrical panel. Would I be OK to wire up the range with the 10 gauge wire since I have a 40 amp breaker?

No, 10 gauge wire can only carry 30 amps. Rewire with 8 gauge as instructed.

You definitely should use 8 gauge wire, copper, also. I would use 8-3 copper with ground. When you install it have the electrician tighten all of the connections. Many do it anyway, but make sure he does.