Eryck,
If you have the holes you've mentioned, I feel sure that you'll have the ones in the floor, too. My early '66 did. However, if I remember correctly, they had plastic(?) plugs in them to keep the elements out, so it wasn't obvious that they were holes. Look for anything in the floor just behind the driver's seat; if it's the plugs I remember, they can be unscrewed to reveal the seatbelt bolt hole threads.
-David Pease
'66 French 230SL
P.S. I did not need to do any welding - they just bolted in. I've never been sure what the welding issue was; maybe earlier cars?