About a 1000 factors to consider but one thing is for sure, a smaller displacement engine will get better mileage if both the larger and smaller are of the same design. If a 265 has the same internals as a modern engine like modern oil, ignition, low friction rings and valve train, and run at a higher temp like the newer ones then IT WILL get better mileage figures. The 235 has a lousy fuel induction system and exhaust but was a great engine for it's time. With the siamesed ports on both intake and exhaust it's limited on economy improvements. Of course if half pedal will give 70mph with a 350 and it takes 3/4 pedal to do the same with a 265 then throw all the above out the window.

Evan