A 40 amp breaker is not oversized for #8 wire. It is just right.
The National Electrical Code requires a minimum of a 40 amp circuit for ranges of 8 3/4 KW or greater.
Here is the pertinent part of the Code:
210.19(A)(3) Household Ranges and Cooking Appliances. Branch-circuit conductors supplying household ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. For ranges of 8 3∕4 kW or more rating, the minimum branch-circuit rating shall be 40 amperes.
The label on your range would indicate it only draws 6.3KW which could be fed by a 30 amp circuit but if the manufacturer is recommending a 40 amp circuit you should follow that recommendation.
Besides the next range you buy may need more power.