I never flew the DC8, but I flew the KC 135 A and R. The CFMs were heavier so the gross weight was increased, but the fuel tanks still only held about 200,000 lbs. offload capability was much increased due to ability to get off the ground with more gas and much lower fuel burn. The KC135A had some truly scary takeoffs. We just rotated at 1000 feet to go regardless of speed. You usually had rotation speed a little before that though even when heavy. A KC 135R at max weight was a non-event just like most modern planes.