Alvin,
How many miles on the car after changing the
oil and are you sure it was full to begin with. I doubt if loose exhaust bolts would run the
oil down so low. Pull the plugs and look for signs of
oil fouling. If they are clean, then make sure it is full, log the mileage and drive it for a few miles and check your level again. In the rebuild did you change oil pans? If so then possibly your dip stick isn't going to the bottom of the new pan. If it just has a few miles on it and that much oil was going into the cylinders, you should be seeing smoke. Try going down a hill in 2nd or 3rd gear on compression and then at the bottom punch it and watch for any smoke.
Ron