By driving the vehicle minimally and expecting to get it fully up to temp once a month, you're talking about a 50 mile run or so, preferably at highway speeds.
As to oil choice, I'm with LT4, except I'd change it twice a year. Also, I'd keep the gas tank chock full between runs, and change the balance of all fluids at least every other year (brake and power steering fluid included).
If your going to do this and expect it to pay off, you have to do it right. Keep the car garaged and covered, a battery tender on it, and be sure the convertible top is soaked with a conditioner and UV-protectant. Rodent protect it, and inspect it regularly. Create a maintenance program and log what you've done, so you can hand it to the new owner (complete with all receipts referenced as well as notes you've made) some day. You might even contact Ford to see if you can get an original sales brochure.
Those are the things that will bring you the coin down the road. And yes, I've BTDT with a 1964 Mustang....it's a lot of work, but if you can keep at it, it might pay off!