There should be some 5/8" alignment holes drilled on the front (at the firewall) mounting ears on the subframe next to the holes where your bolts and bushings go. Drop a plumb blob to the ground and mark the concrete. Then, go to the front of the rear framerail and there is a 1/2" or 5/8" alignment hole in front of the oval slot. Drop a plumb bob from that one as well. Then just take X measurements from the front to rear to get it square.
There's several other areas you can pull dimensions from as well, but these two are the most common. I also used core support holes for my front points of measurement. This will give you a longer dimension to pull for potentially better accuracy.
Don't overthink it too much. I racked my brains trying to get it exact and spent WAAAY too much time doing it. Remember that your car is 40 years old.
Good Luck.
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