You can Dremel a slot into the heads and make the phillips into slotted. You'd be running into the issue with sparks, though. I've heard that you can fill a tank with water to purge the fumes and make it safe to drill, weld, etc. It'd be quite a job to drop the tank, drain the fuel, fill it with water, do your thing, drain the water, and mount it back up.
The hardware is 4mm X 8mm. (UPDATE 10/01/08: 8mm length is hard to find so I wound up using 10mm length with a few washers on each screw to take up the length). I got them at Home Depot. They had phillips, hex and allen cap head. I went with the cap head
because I think an allen will be easier to grab than a phillips if I have to do this a second time but I'm having second thoughts about them being plain steel. I want to check a real HARDWARE store and see if I can find stainless in case I ever have to remove them again. The stainless will corrode less and should be stronger to reduce the chance of snapping them while trying to loosen them.
Edit: Oh, here's the Home Depot barcode: 0 30699 82558 5