I have always had a problem with the restriction of a dual shoulder belt harness for general driving around. Makes it very hard to reach something not right in front of you. I went with a retractable Shoulder belt pivoting on a mount on the roll bar.
I have std lap belts for the rear seat but I put a cross bar just above the floor between the seat frame legs and attach the belts to it. Then I removed the silly clip in rear mounts and changed them to a 1/2" G8 Stud that bolts through the floor with large thick backing washers. Takes about 4 min to put the seat in but its in as well as any seat can be and the belts have a solid anchor yet remove the seat and the belts go with it.