Here's a couple things on it, or how I did it. Find four points on the frame, 
one near the back of the car and one about three or four feet in, on both sides 
of the car, plumb bob down to the floor, mark the frame and floor.

That way if you move the car you can always get your points back.

Plumb bob down from the center of your outside supports, one is a frame support 
with a cage nut in it.

Move the cage nut into the center of the support, and plumb bob down to the floor, 
mark it, same on the other side.

Mark these good on the floor, they tend to want to disappear, because you're crawling 
around under there rubbing them off.

Now find a front and back point on all four supports, that would be the two outside 
and the inside gas tank supports, plumb bob down to floor and mark.

I also measured down from the bottom of the supports to the floor, then you don't have 
to worry if you have them level, of which you will wonder about that when you put them 
back in.

Take some threaded rod, and get some long nuts, there probably about an inch long, 
cut your threaded rod about 3/8 of an inch shorter then the height from the floor 
to the support, then put the nut on, and move the nut up and down, you can get it 
exact like this, but you will need a cheap post level, just a small one will work, 
go back to your points on the floor and level there, and write them on the floor or 

Now you have all the dimensions of the original trunk, you will like having this, 
it's a little or big security blanket for us amateurs

The other thing I run into, is on the back of the trunk, the ribs in the new panels 
wouldn't line up, a couple would but several of them wouldn't, but the front ones did.

This is easy to fix with a little patience, I have it on my website, practice with 
some scrap pieces, and it will work.