Service or owner's manual?
If the house is as old or older than the unit it will likely have a 50-amp braker going to it unless it has been rewired. I was not able to get the wattage of the elements on the parts look up but a high average of an element of 3500 watts will give us an amp draw of 15 amps. If you are running both ovens it will be a 30 amp draw.
Are you blowing the braker? or poor heating?
You may have to run a sub pannel bellow it. I would set it up for a 50-amp. This way it will cover anything on the market. It is likely that the W256W, would have told you to have the 50-amp run to it. Most double wall ovens and freestanding ranges of that era were all 50-amp. Some of todays will only need as low as 30-amps and most are the 40-50 amp. Run it for 50-amp and you will have no problems.