If you are going to do this bed, if you are dead set on doing it (it is a biggie) you are on the right track. You can continue to stud up those low spots and tap down any high spots you can refine it a more before you apply filler.
What you want to do is break it down into small enough steps that you can do it. That is the cool thing about this stuff, most anyone can do it if you break it down into small enough steps.
What you need to do now is get a long board and run it over the top of a small section with some fine sand paper, 120-180-220 it really don't matter. A vexon metal file would also work, but I doubt you have one, so a "long board" an 18" sanding board like this will do the trick.
Running this over that primed metal will show you the high and the low spots. You can keep pulling on those low spots with a stud welder and taping down the high spots until that metal is darn near perfect if you want, you are well on your way to that!
This is a very cool tool for doing this, instead of a slide hammer to pull on the studs like the stud kit comes with, this allows you much more control.
There are zillions of places to get one but here is the first I found on line. http://www.autoparts2020.com/rsdev/p..._HDR_ID=103068
After you do that then we are talking about applying and sanding filler, but that is five or ten steps down the road. Stick with where you are at a little while longer.