The 420mm Apo Nikkor will also cover. It's usually cheaper as most people don't know it covers 8x20. It is possible to put it into an Ilex 5 shutter if you don't want to use your hat.
I use the 600 Apo Ronar. Very sharp high contrast lens. My 300mm lens is the 12" Metrogon. While it gives nice images, it's heavier than hell so I don't think you should go there. I had to structurally beef up my 2D conversion so it wouldn't dform under the weight.
It can be done, but HP5 can be difficult to get a high dynamic range with. FP4 is more receptive but it is slower.
Your camera looks great. Good luck with it.