Sorry, Gorp, I missed the part about the GF's oven lacking a broiler.
Basically if you have a two burner oven (top broiler, bottom bake), you take one route when it comes to baking stones and if you have a single burner (bottom only), you take an opposite direction. What works in one setting will fail miserably in the other.
The broilerless setup requires an oven within an oven scenario. A quarry tile 'ceiling' on one shelf, with foil blocking any openings where air might get through along with a poorly conductive stone for the hearth- quarry tiles or fibrament. With the top of the oven isolated, the oven probe will be removed from the picture and the peak temp will get quite a lot hotter than the dial. This method requires careful monitory as the lower compartment can get quite hot and it requires an IR thermometer to monitor temperatures.
And, bear in mind, this is all to achieve a 4 minute NY style pie, not Neapolitan. You'll never get a Neapolitan bake time in a home oven without a broiler.
Even with a broiler, Neapolitan is highly unlikely. Gas broilers tend to not put out enough BTUs for top browning in 90 seconds and only a very small number of electric broilers have the necessary wattage/coils/passes to do Neapolitan as well.
Since you have a gas oven, I'd give up hope of producing a Neapolitan and settle for a 4 minute NY pie. Once you got that route, it gets SO much easier. 1/2" steel plate positioned on the top shelf (or the shelf below) will do the trick nicely. Get a square piece that fills the entire shelf- from the back wall to almost touching the door, with room on the sides for air flow. This will give a you a big target for launching and will allow you larger pizzas- for NY, bigger is better.