What length rear end fits a stock plymouth Barracuda?

Something to ponder.

You have a 10" wide wheel, backspace on one set is 3" and the other is 8".

The rear end that will properly center those two different wheels is only 10" total difference... Almost 1 full foot. So the correct answer to the original question is...

Don't know!

Which gets back to my first response, depends on the wheels.

Had you asked what factory lengths were, that's a different question and easy to answer.