Rivetting Fuel Sender?

[Bonzo]

If you are concerned about the tanks thickness you could always try using some captive fixings similar to these



Here's an ebay listing for the ones pictured.

Captive fixings & screws

There is a large range of sizes & types available.

Dare say the donor car had a fair few

The rubber should seal on its own if thick enough, bit of sealant wouldn't hurt, so long as it is fuel resistant & you don't get any in the tank.