Definitely consider the continuous footing. You don't want to do that with piers on a size that long. You might consider 5-6 foot piers at regular intervals and use a trencher to get you a foot wide by two feet deep for the full perimeter.
Jerry's right - you don't want a cave-in. Concrete here in Iowa, for a full slab with a footing wall and piers, would run you $8-$10 per square foot (At 40 x 60, that's up to $24,000 for the full slab). When I built homes, the slab foundation was often my largest single expense.
To set it up square is not a problem. You can probably find a YouTube how-to for setting string lines for concrete forming. You'll set stakes several feet outside your perimeter at the corners, pull a twine incredibly tight. You can measure each side and the diagonals to ensure it's square/rectangle. Then, you can measure and set more stakes outside of each location of your anchor bolt needs - run twine across here. All of this twine should be 1-2 feet off the ground - you'll use a plumb-bob to mark the ground at intersections. With your twine elevated this way, you can run your trencher and auger - the twine will flex enough and return to it's place if you bump against it. You'll also be able to pour the concrete without risk of the twine becoming embedded in your pour. I would definitely re-inforce all of it with steel re-bar, too. I would not be surprised if there were a foundation engineer or structural engineer who would want poured beams between your riser posts, too on a span that large.
Instead of a regular flatwork or slab crew, you might contact a curbing contractor. If you poured the perimeter footing, you could always come back later and pave the interior when you have the desire and the funds.
