If you decide to go the I-70/I-76 route, I'd be more concerned about the steep grade over the mountain on I-470 in Wheeling, WV and the grade on I-70 between exit 6 and 10 in WV. They're steeper than any grade on the PA Turnpike. Personally, I prefer to take I-70 through the Wheeling Tunnel and avoid I-470 altogether. However, the westbound tube of the tunnel was closed last month when I was traveling to Barkcamp State Park in OH and the detour was to take I-470. It was a good test for the Duramax and I was pleased that the D-Max was able to maintain 65 going up the grade.
Yes, the original PA Turnpike was designed with a maximum 3% grade and it's documented on their website via the link below.
The Pennsylvania Turnpike - About the PTC
The text reads:
"Although Vanderbilt's railroad bed was originally planned for a maximum two percent grade, the maximum grade selected for the turnpike was three percent, which an automobile could easily tolerate but was still much less than the nine to twelve percent grades on local highways.
The tolls on the PA Turnpike aren't cheap these days. But the grades are gentle and all the long grades are three lanes on the ascending side. You will also pass through four tunnels while on the PA Turnpike.
I understand that Ohio now has E-ZPass. If you have time before you leave (if you haven't done so already), you might want to sign up for it in your state as the PA Turnpike takes E-ZPass too. I find traveling on toll facilities that take E-ZPass makes life much easier... for many reasons.
I don't have a lot of experience traveling on I-80. Maybe someone else can help.