rear end centerline location measurement - 65

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hey guys, i have to cut out my floor starting 24" forward of the rear axle centerline to start welding in my 4-link, but the axle has been out for a long time!
any ideas of how to locate this by using reference points and measurements from your cars? it's a '65 coupe.
thanks a lot
matt

 

[10secgoal]

You can leave it stock and wait for a measurement, or throw some tires in the and place them where you want them. Drop a bob from center of the rim, square that in the car, and measure forward from there.