I didn't add a 3rd row, but consider the 3rd row seats from the mid 2000's Denali's - they are a split design with seat belts built in. Fit perfectly between the wheelwells.
http://67-72chevytrucks.com/vboard/s...d.php?t=755107
I used 3/16" plate underneath 8" wide x the width of the floor underneath the floor as a backing for the mounts. Would be easy to add another row if someone wanted too the same way since the belt mounts are built in.
One word of caution though - legroom will be brutal. The normal 2nd row has the advantage of a drop off in the floor height in front of the seat, a 3rd row wouldn't have that and you cant exactly raise the seat higher. It would be little kids only lol.