Rear Hub Tightening

[Dave O]

Putting on new cast iron drums and new hubs on the rear. I am using an 18" cheater bar and still the castle nuts cover about half of the cotter key hole. Really concerned about using more force. The flat washers provided by the suppliers are approx. 1/8" thick. Do I just use a thinner washer and move on??

I put in new bearings and seals and the drive keys appear to be properly oriented in the grooves of the axles and hubs.