I seem to remember havin a similar hasstle when i did mine.!
Presume the part No's are the same ??
i think it wer down to the fuel in the tank causin the pump to "float" and cock itself over to one side so it dunt sit level.
have another go at seatin it correctly without the ring on ( you can see whats happnin betta) the when you gerrit to sit snugly in the tank Keep pressure on it with one hand and put the ring over yer other arm ,so you can swap hand if yer like, while keepin the bugger pressed down !