beetle seatbelts

Does anyone have experience of fitting 3 point seat belts into the
rear of a '64 beetle? I've bought a standard kit from an automotive
parts store but the bolts are way too small to screw into the tapped
holes I've found on the rear pillars and under the back seat. Also
my bug is about an inch higher on one side at the front - shocks are
ok though - any ideas?