The vast majority of the 8" Pentacs on the used market were produced for the Air Ministry during WWII and are not very good lenses, so I would imagine that the 10" version, which was not one of Dallmeyer's regular sales, would be a better built lens. That said, I imagine this lens open wide (why else would you buy it?) should cover 5x7, probably without movements, with acceptable but not great resolution.
If you are considering bidding, you should be aware that this lens will be very large and very heavy. It will completely overwhelm your average 4x5 or 5x7 field-camera, and probably will not fit on most lensboards. It would be best on a good monorail or on a sturdy 8x10 field camera (but it won't cover 8x10).
