This doesn't help you, but WHY don't they make the Sporty tank wider? Same old peanut shape (I don't like the new flatter style), but just a bit wider???

I had a '97 883. Loved the tank shape, but kept running out of fuel. An extra 25mm wider each side would make a world of difference, and, from the side, you wouldn't notice the extra width.

Gonzo

You can fit the "custom" 4.5 gal tank from a 07-09 straight on (various part number depending on the paint you want).
No need to change the seat (on the nightster- it is already the seat for the 4.5 gal tank).
The bars from the nightster will also work fine with the 4.5 gal. You CAN buy a kit for the larger tank install - you really only need the gasket though when all is said and done.

You know Dave , It wouldn't be that hard to fit a 95-03 King Sportser Tank, If you want to keep the original look of the Bike, It just a case of changing the rear bracket, Ive got a Tank in stock, call me

Dam I forgot about the Pump