Sketch the cut line corners INSIDE the trace lines, and along the crease in the bottom of the channel. Then place the hole saw upside-down along the corner lines & mark the center. I prefer to predrill the center so the saw doesn't grab when the drill breaks through. The front edge of the hole can be an inch or so behind the welds to avoid the lip of the floor support channel. You can always cut the hole too small, and then enlarge it as-needed. But if you cut it too large, the plate won't cover the hole, so always start small. To maintain floor strength, the plate should overhang the rear of the hole, and the screws should be positioned close to the hole and as far as possible from the plate's rear edge. So cut the rear edge of the hole as far forward as possible to give access to the tank.

A slitting chisel in an air hammer is a FAR-BETTER way to cut the hole, and doesn't require any prep.

Owner caption November 2009 · Photo 2991 of 5000