The seatbelts are actually MCG "Seat Belt Material" along with their PE buckles. It's more "fabric" than using ribbon, although it's also more expensive. I'm going to try to find something similar next time I'm at the craft store.
I do want to add, for those of you who haven't built this kit, it is quite well detailed from the box. Those heater hoses were actually part of the kit, and the interior isn't a "tub", so it has nicely detailed seperate panels for each side. Went together very well, minimal flashing, and they did a good job minimalizing the mold lines on the body. The chassis isn't bad either. I'd definitely recommend this kit. My only complaint is that the "glass" is a little wonky, but you could replace most of it with thin polycarbonate if you felt so inclined.