It's better to drop the tank first. That way, you can see where to put the hole for "next time". Might as well replace the fuel pump this time since you'll have the tank out anyway.
Once the tank is down, measure carefully. You'll need to give room for the fuel pump/filter assembly, any pipes attached to it and room to get your hands in there to undo clamps and fittings and so on. After cutting the hole in your floor, you'll need to fit a hatch to cover it. The hatch needs to be bigger than the hole you cut and you need a good seal. The warnings about carbon monoxide are no joke, since your exhaust runs under the floor too the threat is real. After you give some consideration to cutting the hole, fitting the hatch and re-inforcing all of this--this IS a load-bearing floor after all-- it may begin to look less troublesome just to lower the fuel tank to replace the pump.
In any case-- LOWER AND REMOVE THE TANK BEFORE YOU BEGIN CUTTING. You can just bet on making sparks when you cut through the floor, and sparks and gasoline---- not exactly friends, if you get my meaning. It makes it easier to measure so you know where to cut, too.