Some want the belts attached to the frame, others feel that in a crash, that would hurt you as the seat broke free and squeezed your waist against the seat belt. In my opinion, if the crash is that bad, you are in great overall danger; my belts are mainly for if the doors fly open during a turn.
My coupe's center attachments are as shown in this pic. The belts go between the cushion and the back through a window I cut and trimmed in the curved panel there and to the body.
