As a fellow Pennsylvanian that has stumbled on a similar grove, I'm guessing that's Canadian yew.
Canadian yew is typically low growing and shrub-like, except when it is allowed to grow without receiving any trauma (bigger trees falling on them or being cut or broken). When they find a nice safe place to grow unmolested, they end up looking a lot like the ones in your picture. I will admit that 40' is larger than any I've seen, but is possible and about as big as they can get. The ones I found were mixed in with a dense group of pines that help to protect them. The bows I built from them were top notch. I don't know if it's as good as pacific yew because I haven't worked the one stave of the pacific stuff I managed to acquire, but I would have to imagine it's quite similar. Definitely a good score you came across. If you harvest them, please take the time to plant the seeds if there are any. It's not a tree I see growing wild very often.