I finally installed the rails today !!
I followed the FM instructions for the equivalent product, and it went well.
Here are some pictures of the install:
The stock rails were damaged, so I knocked them back into place with a hammer, and to cover all scratches from these hits and scratches from past bottom-outs; I put some gravel guard before putting the BE rails.
Ready to rock this up
Seats, seat belt lower bolts and door sills removed (you don't need a picture for that)
Then it's pretty straight forward. I used a mallet to push them in place and the hammer to move them as far forward as possible.
A look of the inside, underneath the carpet.
For those with lowered Miata, I measured a loss of 6.2mm (¼") of clearance.
I hammered the front part to smoothen the front edge in case I (and probably will) bottom out somewhere. Obviously, the outter corner didn't move much under my hammer attacks.
I thought that the stock line bracket would remove some more clearance if I used metal screws as suggested.
Instead, I removed the line retainers from the two brackets, and just snapped the lines in it; like the other retainers.
Then I moved the lines as far up, one line rubbing on the bolts. That made all the retainers stuck to the rail, so the lines aren't going anywhere.
I saw on FM instructions' photos that there was 4 lines, so my trick wouldn't work on these. On mine, NB with returnless fuel system, I had only 3 lines so that made just enough space for the bolts to go between 2 lines.
Thanks again Travis