My tank is behind the rear axle. If the float level drops enough to open the needle and seat, the remaining fuel still should start the engine. The accelerator pump may be weak causing the long crank, does if have any flat spots when driving?
Like I said above, rebuild kits are cheap for that model, so thats where I would start. I would also set the float to the high side of the spec sheet.
As for a test, remove the fuel line off the carb and replace it with a piece of hose coming from a fuel jug setting on the fender (about carb level). Get the engine running again to clear any air from the line and seal the fuel jug with a rag stuffed around the hose. Now let it set for the same amount of time as you have before and see how it starts.
Joe
