I would agree check with local Law Enforcement. I went down to the State Police office and asked about the Car Seat in my CJ7 and attaching it with the harness or regular seat. There was no law against it in (in VA at least) and as long as I had the proper carseat for the child. Everything was good.
The only point the officer stated was that each car seat today is designed with 3 points to hold the seat in place. The car seat I choose for m the CJ has a 5 point style belting for the child and a 3 point to the car. Check your car seat and see what the instruction say for attaching it to the car with out the common factory 3 point tie ins of modern cars or the alternate instruction for older vehicles. Mine had slots to feed all the belts through for passenger or drivers side.