Air National Guard would be about the best possible way to build time for FEDEX. You will want to pick a unit which flies fighters or heavies (tanker/C5/C17), as that sort of time is valued by the cargo operators. FDX does like fighter types.

But very few people get to pick the major airline they work for, and if they do it's usually by luck. You'll need to be open to FDX, UPS, and certain other cargo carriers if you're determined to fly boxes.

You really should be open to all upper-tier legacy airlines.