On little thing that I would change in Careless's design.
I would make the ribs stick out of the rail a good 2 1/2", for 2 reasons.
1- With the ribs sticking out this much, it's going to be easier to line everything up. *Position your rail where you want it. *Use a scribe and mark each corner (on both sides) of the ribs. *Remove the rail. *Joint the dots with your scribe. *Cut the lines as mentioned by Careless.
2- Now, with that done, put the rails back on with the ribs sticking out trough the top of the floor. *hammer the ribs flat on the floor. * Weld the ribs on each side. *Weld the rail's flange to the floor.
This way you're sure the welds'll be strong enough and they won't pop under load. As it's really hard to weld the tip of a sheet metal part, trough a slot without overheating it. If you overheat the metal, you can be sure those wels ARE GOING to pop under load.
This way, it'll definitelly be stiffer.