Wiring Stove Top and Built in Oven
I'm not sure on this one cause you do not give enough details to be positive. First if the 6 awg wires go to a single junction box or two junction boxes and the appliance pigtails connect to the 6 awg then that part is ok. It's also most likley the case.
But if the 6 awg goes to a junction box and there is tap wiring leaving that junction box to junction boxes serving the stove and oven then those taps must be at least 20 amp rated but also must be sized to carry the load of the new appliance they serve.
Second the 2008 NEC code article that covers this is 210.19(A)(3). You may tap a 50 amp circuit. That's how it reads. The NEC handbook explains that as only a 50 amp circuit not a 40 not a 60. Up for clarification in my opinion.
Anyway I choose to take the article for what it says in the handbook. So I advise to install a 50 amp double pole breaker. Cooking appliances have very diverse loads and would likely never be drawing their full loads at the same time so you should be fine with a 50 amp circuit if you so choose.
" One nice thing about the NEC articles ... you have lots of choices"