A 450mm lens covering 800mm is 83°, and a 600mm lens is just 67°. With that reasonable angles, you have far more choices then the two lenses mentioned above. If those angles are acceptable, I'd choose between the most desired pictorial qualities (such as the out of focus rendition) and the availability/price of the glass.
Protar IIIa 407mm f/9
Dagor 420mm f/7.7
Dagor 480mm f/7.7
Protar IIIa 505mm f/9
etc....
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