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Depends. If it's a 1966 F100 4x4 the cab is the same overall as the 2wd trucks. If it a 1966 F250 4x4 then it would be quite different than the 2wd trucks. The F250 4x4 has taller steps inside of the door, different floor and firewall - the hole where the steering column exits is different. The battery is underneath the passengers floor board with a removeable cover to access it. Different heater housing. And the front cab perches/pedestals and mounts are different.
Easy way to say it is 1965/66 F100/250 2WD and 1966 F100 4WD (trucks with coil front springs) used the low step cab and all other series and years all the way up to the big trucks used the tall step cab and front leaf springs.
Custom Cab didn't change the shapes of the cabs at all. It was just a package of options.