I think you should have started a new topic, these don't really belong together at all. That said, hot issue for me and I have a few old cars (and two in car seats). A few thoughts:
1). Buy a smaller car seat....there are plenty, don't let the wife tell you a certain brand is needed to keep up appearances, they all do the same job. I've got 12 seats and each set is meant for a different car for the best fit. I've got mostly gracos for the defender and others but I use coscos (40 bucks from Walmart) in the 911 because they were by far the best fit.
2). Lap belts whenever possible. I have added many sets of lapbelts to cars where I had existing mount points that could be utilized. In the defender that meant I had 3 point and separate lap belts in the same seat depending on if I had the car seats in there or adults.
3). This seems like common sense but just in case. You should be IN the car with your body weight in the carseat (knee) as you are tightening the F out of the seat. Thing shouldn't move a millimeter even with just a lapbelt if you do that.