Well you can go 3 ways
1. the way you are going
2. Make a 6" hole and put deck plates, this way you can adjust them or what ever if there is a problem.
http://www.westmarine.com/webapp/wcs...classNum=12946
3. Use seat mounting kits were they have a SS toggle bolt, you drill hole stuff the T in the hole and pull up, insert bolt, they hold seats in place for years!
http://www.westmarine.com/webapp/wcs...7&classNum=563
I think you are using the hardest way of all! I like option 2 the best, but that does leave a cover on the deck all the time.
In order I would use options 2, 3, and 1 last.
Good Luck!