I posted this in Body/Interior, maybe this forum could help me out too?

I'm trying to install a locking rear storage cover in my car that I got from a junker. I've read the tech article on this site, but it doesn't offer much advice on lining up the cover properly.

Since my car never had a cover, it does not have the welded plates on the floor just behind the gas tank that the cover normally bolts too. My plan is to weld on new bolts straight to the floor (being careful about the tank of course) since I think it's a little dangerous to drill new bolts through the floor there and possibly poke the tank.

Any advice on lining this cover up properly? In pictures, it appears level with the rest of the carpeted floor in front of it. My car has no carpet or interior whatsoever at the moment, so it's difficult to guess what height I should weld the bolts at.

Anybody have any advice, or even better, some measurements of where their bolts are? Thanks!