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.