This is the reason for a frame table!! LOL
I'd set the car on leveled jack stands (you can use some thicknesses of steel sheet etc) - get it level fore and aft and across... I like a 6' level -- and use some lengths of steel tubing to bridge the gaps... (borrow these if you need to). Or get yourself a laser level so you have a fixed point to measure from - these have become quite inexpensive and can be used for lots of things.
Set the car down on them at the same points of contact side to side and front and back... See if the car sits level or does it rock etc. Then I'd drop some plumb lines down to tape on the floor. See if these points are "square"... same distance ACROSS in an X pattern (like squaring up anything)... and that they're the same width and distance front and back... if you can find that square.... Then connect the dots! From there you can pull a tape for take off measurements.... again using a plumb bob to a point on the floor.... The sub should be square to the other square plus or minus a 1/16th...
Using the taped points on the floor - you can pull repeatable measurements and you can reposition various parts (such as the sub) by simply realigning your floor points and the plumb bob...
Make sure you don't set up your jack stands in a place that will interfere with bolt holes etc that you'll need to access. Use tape to mark the floor where they sit so you can move them or sit them back in place if you need to reposition for some reason.
Remember that these bodies were done on an assembly line and the tolerances "then" weren't so great! Don't be surprised to find you body "off" - wheel openings side to side at different heights - or ground to various points off... These cars are designed with rubber bushings at the suspension points - and the subframe was on rubber bushings - and the body mount positions are "close enough"...
Your alignment is designed to be able to make adjustments... but you still want to start off with everything as tight of a spec as possible.
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