Do you have or can you borrow a copy of the Lindsay Porter book "Restoration Manual - Mini" (published by Haynes)?
If you can get a copy of this book, check the pictures on page 136 and 138. There are pictures of the lap belt anchor nut being attached to the inner sill. I THINK the size of the nut is 1/2-20 (1/2 UNF). The only way and time you'll be able to add that nut is during outer sill replacement. The approximate location of these nuts on my car is/are 2-3/4" forward of the rear companion boxes and about 2-1/2" DOWN from the top (vertical sheet metal) edge of the door step.
Adding an upper anchor point for a shoulder belt will be much more complicated. Frankly, it may not be possible or advisable. That nut is inside the door pillar and I can't image a way to safely open that up to weld a nut in place and still have a safe situation. However, you could possibly cut a pair of complete roof pillars off a later car and weld them in place (complete). This might be easier to conceal as well.
The lower center anchor points aren't shown in the books that I have. However, they are clearance holes on either side of the tunnel in the floor. They are located 16" behind the floor cross-member just about where the up-swept fillet joins the tunnel to the floor. On the far side of these holes are jumbo, toughened fender washers and large nuts. There are no weld-nuts here.
For this period in time the belts could be either Britax or Kangols (with the magnetic latch).
EDIT: You may also want to consider running this thread in the paint and bodywork pages of this board.
Edited by dklawson, 21 February 2008 - 12:54 AM.
