If this is for a paying customer, And he's willing to pay the additional cost, I would find someone with a thread milling machine. Be a heck of a lot easier than trying to singlepoint such a very coarse thread as this. If you're in the Dallas/ Ft Worth area, I know of a place up there I used to use many years ago for thread milling lead screws. They are still in business, I don't know if they still do thread milling, only one way to find out.
But if this is for yourself, Follow the advise already given, a follower rest is a must. The jaws may/will have to be modified with some kind of wide jaw to straddle at least two threads at each position of the jaw. Think this out a bit or it will eat your lunch in a heart beat as you get into it!
My thread calculator says the thread has a nominal depth of .210" and a root width of .280". And of course a pitch if .666". Will your lathe cut this pitch?
I would cut this thread in steps using a say roughing tool ground to a 4-pitch Acme threading tool. Since you are cutting stainless steel, that's nightmare no. 1. I would work on getting the thread depth down to about .125-.150 deep first. Sidestep the threading tool, both directions to get the crest width of the thread close to .300". I forgot to tell you to set your compound at 90 degrees to the cross slide or parallel with the work piece. Once you get it about .125-.150 deep, work on getting down to around .200-.210 deep. Once to depth, start side stepping the threading tool to get the .280 wide root width. As you get to the proper root width and depth, use a freshly ground threading tool with a nose width of about .150" wide. Any wider width is probably going to induce chatter into the thread. You don't want to do that. What ever you do, don't use a full width threading tool. It takes a pretty stout lathe to handle a threading tool to full width. Especially cutting stainless steel. Be sure to use a good cutting oil such as Mobil Mobilmet 766.
Ken