Dexron III can be used and you can also use the newest Dexron VI which is backward compatible and provides more protection than Dexron III.
In the fluid chamber
The original base price was $2,618 for the buick special 2dr. convertible in 1965. The performance wildcat in thee same configuration was $3,431 for comparison.
Automatic transmission fluid
You add transmission fluid at the exact same place you check the fluid level. Do not overfill!
transmission fluid
Use Ford Type F fluid.
Type F
80W-90
Type F transmission fluid unless it's been rebuilt. Then it would be Dexron III or whatever fluid the builder recommended. Mine it was Dexron III
Do NOT use transmission fluid in your power steering. It is designed for automatic transmissions, and a few rare manual transmissions only. (1965 Plymouth "Slant Six" manual transmission used automatic transmission fluid). In most Chrysler products made after about 2002, ATF is what they use for power steering fluid.
Using the right types of fluid in a car will help maintain the car's running life time. A 1965 Ford Ranchero takes universal ATF.
Ford automatics of that era use Type F fluid. Incidently, so does the power steering unit.