Starting to rehang nose on my 69,and noticing that some body panel gaps are all over the place. Had removed chassis to do resto and replace mounts,when I reinstalled I used a straight edge off the rockers to measure squareness, but now I'm looking for some additional ideas to check square. Any help out there to set me sraight.

Find 2 points, one on either side, behind the subframe that is an equal distance from the rear of the car. Measure from that point to the opposite side of the subframe in 3 different locations, rear, center and front. Do the same on the other side. If you tied a string at each measurement location, you would have 3 X's of differing lenghts.
Each of the measurements should be exactly the same as the opposite measurement, starting point to rear, starting point to center and starting point to front.
When you are sure these measurements match up, you will have to adjust the core support from side to side at the front of the subframe to get a proper sheet metal fit and this might have to be done after you have hung the sheet metal.
It might help if you don't tighten the fasteners when you install the sheet metal the first time. Make sure it all lines up correctly and then tighten the bolts.