Trying to find someone "knowledgeable about gas ovens" but even if I do I don't think they will match up to the combined knowledge here.
Richard, while there is a great deal of combined knowledge here, I don't think any of us has ever built a gas oven from scratch. At least I don't think any of us have (Jeff, have you?) Unless you have someone with gas oven building experience at your disposal (building more than one oven), I would either proceed incredibly carefully or not at all.
I've analyzed as many as 60 parts schematics for deck pizza ovens and not a single one has ever had top burners. These bottom heat configurations go all the way back to the 50s. If, for the last half a century, no one's come along with a successful top burner deck pizza oven, there has to be a good reason. It's most likely just a cost issue, but there's a slim chance it might be related to safety. If you can find that person with oven building experience, go with the top burner. If you can't, I would discourage it.
SO many ovens have top heat issues. It's almost the norm rather than the exception. When you go shopping for a Weber for an LBE, you should swing by the pharmacy for a lifetime's supply of aspirin to go with it. WFOs generally require a virtuoso to manipulate. You're not building those kinds of ovens, though. A properly built bottom burner gas deck oven has absolutely no problem churning out perfect NY style pie after perfect NY style pie. The Marsal MBs make flawless pizza. You can mirror the thermodynamics of a Marsal MB. It's just about achieving enough BTUs, a small enough vertical space, proper deflection and a firebrick ceiling.
If you get a gas oven expert and go with a top burner, then I'll share my thoughts on top/bottom btu ratio. Until then, though, my recommendation continues to be 120K+ on the bottom.
One thing to consider re; the steel, is that you'll need some stainless angle steel in weight bearing roles in hot areas, but, other than that, you can use very light gauge stainless sheet in the hot areas and, for the exterior, regular angle steel and very light gauge steel sheet.
I'm curious, can you purchase a gas BBQ grill in Burma?