Here is my solution. I have an 81 with only lap belts and whatever I did could not get it tight.
Installed metal brackets in the seat belt holes, which are designed for 6 point harness anchor points for the front seats.
The WAYB Pico car seat fits quite well
Used a ratchet strap to hold the seat in which allowed me to get it REALLY tight.
Even though I don't have anything for the top anchor point, the car seat doesn't budge at all. I use this for my 5yo daughter.
