As long as when you move her forward facing she is in a 5 point harness seat you should be good to go. Vehicle choice, where and how you drive all come into play. Many many times I chose Sooner Road driving from Norman to Moore rather than I-35, because the route is slower and safer with kids in the car. A seat anchored solidly to your vehicle, properly adjusted harnes belt, the chest buckle should be at sternum hight, the actual over the shoulder straps should not be hanging down from the slots in the seat, the should be even or lower than your child (If I remember correctly- mine are now in booster seats) your child should be safe as possible. Like previously mentioned local fire departments can ensure your seat is installed correctly. The weight issue and if they are strong enough to hold their head up is a big deal on forward facing.
I agree with this as well, I feel that my kids are safer in the center of a 8500lb Ford Excursion, than in a Nissan Altima or something similar. We do a lot of highway driving as well, both of us drive a little more than 30,000 miles a year.