Post a picture of what you have in the car from the underneath. If you're missing the supports I mentioned, you've got bigger problems
But per your request for measurements. Here is a pic of the underside of my 71 hardtop (currently on my new lift
).
The floor pans have been previously replaced but you can see what Alan was talking about (circled in yellow).
I measured the dimensions and added them to the pic. I hope it isn't confusing. Please keep in mind these are replacement floor pans so it might vary by a 1/4" or so because of placement/installation but gives you an idea. You should have the square plates with holes already in place.... I hope.