Since AACA judges cars "The way they left the factory," and CCCA judges cars "The way they were delivered to the original owner," you should expect a point deduction for any non-authentic hardware that did not originally come on the car. I've never seen a car with factory "D" rings installed. Of course, if you could provide factory documentation to the contrary for the car in question, then there would be no deduction. You'd need to bring that documentation with you to the judging event.
My recomendation is to use the nylon axle straps that are available from most trailer manufacturers and suppliers. You simply loop them over the axle and connect your tie down strap to it. They are easy to use, and can be purchased in a variety of load ratings. If for some reason, these stock versions won't fit your car, I'd suggest finding your nearest Aeroquipt dealer and have them custom make the appropriate straps. I've done that myself. They were easy to put on, and not all that expensive.