It may be too late, but for what its worth:
Be mindful of the max bellows draw of a camera. I could not find any info on the 45a, but this is for the 45aii:
<b>"full 324mm extension " ... "standard design 300mm can be focused to ten feet for complete versatility in landscape work."</b>
In large format, there is a big difference between a 'long focal length' lens and a true 'telephoto' lens.
A 'normal' 300mm lens will need about 300mm of bellows draw focused at infinity. It will need more extension for portrait shots and much more for close-ups. Up to 600mm for 1:1.
A true 'telephoto' requires less bellows draw. So a 300mm tele only requires 200mm bellows draw. (for example) Tele's have some drawbacks though. Using movements are harder to use. They are heavier and have less coverage.
So if your camera matches the specs of the toyo 45aii, you will only be able to focus as close as about 10 feet with a normal 300mm lens. Also, at this extention, the bellows may be fully extended, which could cause some spring back while focusing and limiting movements.
I would recommend instead something like a 250mm commercial ektar or 240mm g-claron.
One thing I would like to add is that I find focusing with a dimmer lens just fine in the longer focal lengths. I have a 360mm Artar that I adore and is a breeze to use.
The artar is not a tele btw. Large format lenses will usually explicitly state if they are tele.
In any case, the 300mm Xenar is a good lens if it works out for you, all the better. Since you want to use it for landscapes the bellows extention issue will not be a factor.