I know people have swapped seats with the built in ones, works well and simple. As far as using the stock points, may be a pain. This would also be good if you plan on having people go in and out of the back as they will get tangled in the belt easily.
I just recently installed 3 point belts from Morris Classic Concepts, and they bolt directly to all factory anchor points. There's no drilling or fabricating any mounting plates - just remove the old belts and install the new ones. The look is factory-like and very presentable. The only thing I don't particularly care for is the rather large retractor housing that mounts to the floor, but I'll overlook that, considering the extra safety feature that these belts offer. This belt system is not cheap (almost $300 for the set of front belts) but well worth the price IMO.