I have used the Fuji 6x17 for many years, and I have the 300mm Fujinon.
My solution to your problem has been to purchase the dedicated Fuji 6x17 ground glass adapter. This adapter takes the place of the film, with the back swung down, and allows you to see the image upside down as in a view camera. A small pop-up shade let's you use a loupe without a black cloth. To see the full image you do need a black cloth however.
I don't see how else you can be 100% sure that your image is sharp without this accessory plus a ground glass loupe (magnifier).
With the wide angle lenses, and when shooting at infinity with no foreground elements, one can use a small f-stop and get good sharpness throughout, even when handholding the camera if the shutter speed is fast enough. But with the 300 the shutter speed need to be rather high (1/250th minimum) which means the aperture will often be fairly open. Plus, it is virtually impossible to precisely set the distance since focusing with a telephoto is critical.
Of course, you have to have a tripod with the groundglass adapter. But then again, if you are using the 6x17 a tripod needs to be part of your kit as Doug mentioned. For faster, handheld panoramic photography I would recommend the Hasselblad xpan.