Agree on opening the fill cap to equalize pressures. In-tank fuel pumps use a brushless motor, eliminating arcing, and are not always fully immersed. gasoline level senders, nevertheless, possess low amperage voltage that is directly exposed to gasoline and vapor; yet, any possible damage to the sender that may generate a spark is insufficient to cause ignition.
Explosions arise from the accumulation of explosive vapors; hence, it is crucial to mitigate this risk. I like the computer fan; however, it seems to be expelling air outside. I would have it draw in air, inflate your 'balloon', then position a tiny vent hole opposite the blower to expel any accumulated gasses.
I have installed almost 1000 furnaces. Initially, I would operate the new installation to purge air from the new natural gas line, followed by using a torch along the gas line to detect natural gas leaks. Occasionally, a fitting requires tightening; nevertheless, on two occasions, a sand-cast fitting exhibited a pinhole that emitted a flame around the size of a cigarette lighter. No detonations due to the absence of buildup. Hooray for Chinese workmanship.I consistently repurpose outdated fixtures wherever possible.