Squaring up front subframe on a 2nd gen F-body

[Doug1]

I need to square up the front sub-frame before welding in the new sub-frame connectors. The rear end is not underneath the car so I need to utilize some method to square it up with the rear frame rails .
I know to measure from corner to corner but what would be the best place to measure to on the back rails? I've marked in red what I think is a good area to use but feel free to chime in if there is some other marker that would be better to use.
Below are pictures for reference.
https://static1.pt-content.com/images/pt/2013/08/3a60cbe72c474d969b32284ab954e046_zps3ca7-1.jpg
https://static1.pt-content.com/images/pt/2013/08/9a10a60e98684acaad0c11810765becb_zpsca21-1.jpg

[Skip Fix]

Getting both upper locating holes exactly lined up will get you pretty close. There is a hole by each of the middle bushings in the frame and body for that. I forgot what size deep socket fits in there.