Suspension mounting points should be as close as anything. Locating holes in the subframes and rocker pinch welds as well. 60s, 70s and early 80's cars are all over the place. If the car has been hit or had sheetmetal replaced, good luck.

Use stands under the tires or support the car at multiple places including suspension mounting locations. Make sure the doors all open and close right.

Roll bars and cages require you to use your eyes as well as your tape measure and level. It is not uncommon to have to make something a little crooked to make it look right in the car. Work off of the center of the car to build the main hoop. If it's a pre-bent kit, you're kind of stuck making it look right as bent.