Back in my 8x10 days I also went for the last 2-5% of image quality, practicality (weight/bulk) be damned. I owned the full Sironar-S 8x10 trifecta: 240, 300 and 360mm. Magnificent lenses all, albeit a pain to haul around in the field (they required a separate carrying case of their own, aside from the camera backpack).
The Sironar-S 300 is a wonderful lens and a good anchor to build an 8x10 lens set around. The Sironar-S 360 is a bit less practical in the field, primarily due to its larger size. Going longer than 360 involves weighing various trade-offs.
I have never owned a 480 Sironar-N or any of the APO Ronars, but I have owned a couple lenses that you may want to consider: a 480mm Schneider APO Symmar L plasmat (for maximum image quality) and a 450mm Nikon M tessar (for practicality). The Schneider has a distinct look compared to the Rodenstock: it has arguably smoother (some have called it creamier) tonal transitions with a smidgen less edge contrast, although overall clarity, color saturation and resolution are similar. I used to speak of the Schneider as being "sneaky sharp." It is also significantly smaller/lighter than the Sironar N, despite having the same rated image circle. The Nikon has slightly less overall contrast but, darn it, is much much smaller and oh so practical. Its rated image circle is also conservative, and as a practical matter I found it to have a larger usable image circle than the Schneider, even when used for enlargements. If you are shooting architecture you may actually run out of image circle with the Schneider, and may find the Nikon to be a more practical choice.
For the 600mm focal length, it comes down to how much extension your camera has, how long your arms are, and how much you need front movements. If you have sufficient extension and long arms, it's hard to go wrong with the 600mm Fuji C. Just be sure to use a hood with this lens to avoid bellows flare, as it has a very large image circle. There are also two telephoto choices: the Nikon T (commonly available, relatively compact and practical) and Schneider APO Tele Xenar (relatively esoteric, larger/heavier and very expensive). The Nikon has a bit less contrast and a smaller image circle than these other lenses, although camera movements may be less important with such a longer focal length. The Fuji and Schneider provide the best absolute image quality, if that is the sole objective.
There is also a less conventional, more controversial option: the Cooke XVa convertible lens (311, 476, 645mm focal lengths in one lens). The Cooke coatings are arguably superior to the Sironar-S, but the Cooke 311 configuration has a smaller image circle and for the life of me I could not get satisfactorily sharp images from the longer focal lengths (which require hugely long camera extensions). On the other hand, some folks swear by this lens. YMMV. It is very expensive.
On a separate note: in my personal experience, I found 8x10 long lens landscape shooting to be ultimately disappointing. Many factors conspire against image sharpness: shutter kick (all the 600mm lenses above use large Copal 3 shutters), camera stability (a substantial camera with multiple tripods is often necessary, as is an umbrella to deflect the wind), and local atmospheric conditions (haze, convection currents). Even if camera stability was assured, I often found myself getting high resolutions photos of haze, even when using a cutting filter. Of course, I live in the SW United States, where haze is a major problem. Again, YMMV.
There are tons of posts about all these lens choices over at the LF camera forum.
