Most engines made before the mid sixties had cylinder walls thick enough to stand a 1/8 overbore.
I haven't checked the Chrysler 6 specifically but believe they can be bored this much.
A 265 bored 1/8 over would have a bore and stroke of 3 9/16 X 4 3/4 and a displacement of 284 Cu In.
This would be the largest practical Chrysler flathead 6.
