Reuse aluminum wire for basement Sauna heater
Honestly, you're probably better off leaving 40A service to an electric stove in place.  It's a selling point when you go to sell the house, especially when it's a competent 4-wire circuit, for people who might want to go back to electric, or do a best-of-both-worlds gas range and electric oven.  Since it also has gas, that takes the sting out of the aluminum wire for those who are bothered by #6 aluminum.  

If it's convenient you might drop #8 aluminum or copper straight down from the stove receptacle to the new appliance. You need to make an Al-Cu transition anyway, inside the stove box is as good a place as any.  

A 6 KW heater is 25 amps. That must be derated by 125% to 31.25 amps.  You cannot put 31.25 amps on a 30A circuit unless the device is UL-listed and the UL-approved instructions tell you to do that. If they do, feel free to use #10 copper for the final-run to it.  

You cannot put aluminum wire on the device's terminals; they are not listed for AL.  So you must pigtail with a legal size of copper wire. You can splice from #6 Al to copper with a "MAC-Block" insulated splice, inside a junction box.  That may require mounting an additional junction box.  See how that's enough trouble that you might be better off just making that splice in the oven box?