The relief valve won't really care what your case is. What is the relieving scenario? You're going to get an enormous amount of flow at 43,500 psi regardless of how big the valve is. In the past at our company, we've used the RVP series with the published areas by Autoclave, and a discharge coefficient of 0.14. I think the K was back-calculated based on some of the published flow curves in the literature.
Unfortunately I don't know of any other valves that can go up to that pressure. I checked the normal players (Leser, Farris, Crosby, Flowsafe, Consolidated) and none of them even get close to those pressures. Most Autoclaves that I've seen operating in those pressure ranges always have an extruder-style rupture disc, or very small safety head.