Backspace difference?

Stand up the wheel, clamp another board against the other side and measure the width directly through the hub hole.

That said, I measured the backspace on my 67 Barracuda rims at 3-1/4, and I thought they were 5.5" rims (that's what my dad told me once). Thinking about it, 3+ inches would be a lot of backspace on a 4.5" rim. I always thought Mopar rims were close to zero offset.