Rear axle measurement

Looks like you're doing it right to me. That's basically how I built the rear in my 66 Dart. You can double check that measurement by measuring outside to outside on the frame rails, and an inch per side for inner tub clearance and do some math taking into account 1/2 the tire section width and wheel offset (relationship between the wheel mounting flange to the center of the wheel). If the two numbers match up, you're good to go. Keep in mind it's better to be 1/4" too narrow than 1/4" too wide.