Okay I've spent hours looking at maps, interstate histories, and just generally spinning my wheels on this and I really think perhaps we're over complicating this. Could Beth be from anywhere within a 24 hour drive including Canada? Yes. Is it likely she was from more than a 4 hour distance? I don't think so.
We know Beth was found with the New York Daily News. This is a newspaper that is well read but very local. It's not the New York Times, it's not likely to be read by someone not from metro New York.
We also know it is very very likely Beth's killer dumped her while driving westbound on I-80. In 1976 "the Blue Route" (476) wasn't build yet so traveling from Philly or south Jersey seems less probable than it would be today. I-380 which connects I-81 and I-84 was under construction but was likely finished by December. Due to positioning it is unlikely someone on Route I-81 would switch over to I-380 without a reason. A far more likely scenario if you were taking I-380 would be to come from I-84 which connects New England with the area north of NYC. Of course, the most likely scenario is that the perp was traveling from the NJ/NYC area which we all know is straight shot. Alternatively, there is the possibility he could have come from anywhere on I-95 and then made his way onto I-80 near the GWB but the chances of that are decreased by the newspaper indicating he or Beth had been in NYC in September. Also, most people coming from the south on I-95 would choose to go on I-78 if they wanted to head into PA unless they had a reason not to, once again pointing to the perp being from northern NJ, NYC, or New England.
All roads really do lead to the fact that either the perp, Beth, or both of them were from, or at least spent time, in the NYC area. That's just my 2 cents and I could be totally wrong, but I think that's the most logical conclusion.