I'm putting together a 1990 GMC Jimmy for my wife and the 3-points were part of the problem. They had the roof mounted point, but she wanted to be able to take the roof off. Being as it came from the factory with 3-points it is illegal to run 2-point belts.
Best solution I could come up with - mounting mid 2000's Yukon Denali 3rd row seats. They have 3-point belts built in.
I used 3/16" x 8" x width of floor plates under the 2 rows of mounting points -
http://67-72chevytrucks.com/vboard/s...d.php?t=755107
Works like a charm, have 3 seats in the row and has the legal requirement for seat belts.
Suburban 3rd rows might be a bit cheaper, they aren't split like the Denali / Escalade (and maybe Tahoe?) 3rd rows