I have had two 69s with those belts. I did not like them and never used them. My 68 has the proper anchors, but since it was not mandatory in 68 to have or use them, I said good riddance to them. Just my opinion.
If I had your problem and really wanted those belts, I would pull the seats and carpet, throw on a welders blanket and get busy.
I have seen some nice solutions here. I could add mine,but I really like Daves69 in post #28. Perhaps you don't understand how easy that would be. You might need to elongate the existing hole in a vertical direction, and perhaps also a tad in the horizontal; but perhaps a bit of prying would get you in. You can secure Dave's strap in position with any of several methods. The securement does not need to be strong. I would orient the strap in the horizontal direction.
If you distort the sheet metal to get the strap in. it is easy to straighten out afterwards, by using the strap as a base-plate for a clamping tool.
