Squarring the Rear-End after Spring Install?

So, I got 002 and 003 SS Springs for a Stock '65 Barracuda...
I figured I'd Mount them Tight Front and Back and do the U-Bolt's last.
How can I make sure that I'm Square in there before I torque everything down?
I'd hate to do a Donut backing it out of the garage before I get a chance to see the
Height difference they made.
Thanks to All and Apologies for Newb Question.
-eric

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