You may find that you need a brace with a larger than normal sweep for boring a hole this size. They were manufactured in various sizes. A more common configuration for an auger of this diameter would be something like a barrel eye pattern. They are still available new - here
The advantage of these is the additional leverage given from the handle, and they're generally tough enough to drill through oak.
The alternative is a flat bit - simple and cheap. Make up a block such that it has an angled face so the bit starts horizontal, and then clamp the block above your desired hole to ensure your bit will follow the correct direction. Augers or flat bits are not good at starting at an angle, and need help.
There are also modern saw tooth versions of forester bits which will do the job, using power tools, but again, for a typical splayed leg angle, they may need some alignment help, despite their accurate habits.
An adjustable bit is weaker than a solid one. You will find it very difficult to start it at an angle, as it makes contact at only one point of It's sweep. Further, if you are using beech or similar for your bench top, then I'd be very careful. The lead screw is fat, and the cutter is weak. A recipe for a broken bit. A oak-beamed tiber ark is still in existence with a part of one of my adjustable bits. Of course, that's just one instance, but my spider senses were screaming 'bad idea' and they're usually to be relied on. The urgency of completing the restoration, on site, in the woods, cost me the bit, and a return visit.
You can also make your own scraper version to enlarge a hole, or you could go with square mortices - Whatever Chris says in his book. Back in whenever one would have access to a lathe of some sort for the legs, and they would be made to whatever large size auger was on hand. Spoon bits on twenty foot irons existed for boring out water pump timbers and for string on line. Needs a skilled hand to use them of course.