Wow, that is a great looking install - and certainly a huge improvement over no belts at all.
One thing that raises concern however is the belt being passed through the seat frame sheet metal. Under crash deceleration of 3-5 G's or higher . . . those sheet metal slots might act like a knife and cut right through the belts.
No easy fix for that with a stock seat frame in the way. A rolled smooth surface would provide an adequate pass through for the belt.
