Well, that depends, I would probably start with a similar focal length to what you would normally use in 35mm. So if you like the 50mm lens, which is normal, then the equivalent lens in 5x7 would be a 210 mm.

I do not know if the super-angulons will cover the 5x7 format, you will need to check the image circle specs, you will need an image circle of at least 210 mm, to avoid vignetting.

Also as you move into the longer focal length lenses, you will also need to consider if you will have enough bellows draw on the camera to focus at you normal working distances. Probably something in the 300 - 360 mm focal length may be usable for portraiture.