Good call on waiting til the cage is in. The mounting of the shoulder belts is VERY critical!!! Without the cage, and the crossbar in the correct location, the only option is mounting the shoulder belts to the floor, and that is wrong wrong wrong. If you were to get into an accident with the belts mounted like that, you could sustain SERIOUS injuries, possibly spine fractures etc etc. Read up, and install the belts correctly, you'll thank yourself for it later. I see belts mounted the wrong way a lot over the summer, but funny thing is, it's always in a "street" car. For the most part racers know, and do it the right way.
I just hate to see people get hurt for no reason, especially when they're trying to be safe in the first place.