Your burner guy should know where to put the holes for the burner. He understands BTUs, correct? If your burner guy is depending on you to choose where to put the holes, then I think you need to put in at least a couple days reading through the burner forum that Jeff posted a link to. Even if he knows what he's doing and is building you something very close to 120K Btus, I'd still spend some time reading through that forum. You cannot learn too much about inspirators, venturis, and stoichiometry. Instead of asking us questions on building burners, you should be research the living crap out of this and telling us how to build burners. As far as I can see, that's the only way you're going to know that the setup is safe and to be able to sleep well at night.
Or you can save yourself some of that research time and buy a commercial burner. How much were the Weber and Chinese stainless burners you saw?
Marsal does an H-ish shaped burner, but they also incorporate deflection- deflection that isn't shown in the video. If I were building this, I'd probably go with four 12" x 28" stones, with a 24" pipe burner centered under each. You could probably go with an H burner and simplify the deflection with one solid plate of steel, but an array of 4 pipe burners would spread the heat a bit more reliably.
When it comes to safety, I think the most important aspect is making absolutely certain that the burner always stays lit with an incredibly reliable pilot light. These kinds of homemade burners can easily go out, and, if they do, and the gas keeps flowing without re-ignition, you have a gargantuan problem.
