| Jim, take no notice of the Haynes manuel. I recentley took my tank off also to get the fuel pum pout. I didn't turn it upside down, but placed it on one end. There was enough fuel in the tank so that it started to weep out through the fuel cap. The only reason I can think of regarding fuel pooling in the recess, is through the overflow / breather pipe.
The tank has to be vented so that when the fuel level falls air is drawn in to replace it. If this is not allowed to happen. Your fuel tank will collapse inwards.
With the tank being upside down, I think the breather pipe could have pushed fuel out under pressure. I think, but don't tke my word for it. Fumes from the petrol or even the volume of petrol expanding due to heat, could of pressurised the tank and pushed it out through the breather pipe. | 
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