You're shifting too soon. With my '91 3.0, I got the best performance if I was just coming out of second at 45mph. Out of first at around 20-25mph (I think), out of second at 45, out of third at 60, touch fourth, and into fifth.
Reminds me of the first time my then fiance and I took her Justy on the highway after we put it back on the road for her. She could never get that thing over 60mph, but that first trip, I hit 70. She was shifting too soon. At 50mph, she was already in 5th, but I was screaming up through third. That thing was a gutless piece of <censored>, by the way.
The way I understand it, horsepower helps you get up to (and hold) speed. Torque helps you pull (a trailer). The 2.9 peaks at 4600RPM. In fourth, at 45mph, you're probably turning over around 2500rpm. In third, you'd probably be 3500ish.