If you want to keep it, keep it. The way the old static belts used to attach was to the rail on top of the bulkhead behind the seats, so it shouldn't be a problem for an anchor point. I'll be doing the same thing with my SIIA when I get around to it.
I bought an old Rangie for our family car and had a CRAB bar put in the back. That would also be worth looking into. But just take the car to whoever puts anchor points in and ask.
Here's a photo of the factory-fitted nut under the rail where the top of a static belt would attach to. It's actually welded into a small bit of channel that fits into the capping. Something similar wouldn't be a problem for a baby seat anchor point.
http://www.aulro.com/afvb/attachment...1&d=1348132475