We had seats with the base for our first two kids (now 16 and 13) and ended up not using the base portion since the seats also had provisions to mount the lap belt across the top of the seat and secure it in a car without the base. Ended up being easier to install and gave us more room. Luckily my wife is under 5' so the passenger seat always sat all the way up and we had enough room in the back. The Stang was our only car until my daughter (now 13) was 6 month old, so we had a lot of car seat time in it. We used the car a few times with my now 6 yr old twins when they were in infant seats, and having both in the rear was not "fun", but we managed.
As for the seatbelt clips, those are only required for 90+ cars. The 89 and earlier only have the lap belts and they have a built in rachet assembly that locks the belt in position and hold the seat securely.. But of course the lap belts are not as safe when they outgrow the car seat and can use the booster seats shown above. That is why I installed 90 model belts on my car about 10 years ago. But I kept the locking lap belts also just in case I need to mount another car seat to drive my niece or nephews around.
Good luck with it. My kids have really enjoyed riding in my car, especially the twins! "Go fast Daddy, go fast!!!" LOL!