Installing a harness

I'd love to replace the lap belts in my 68 Cuda for something more safe but can't think of a way to secure the over-the-shoulder section since I don't have a cage/bar in my car. My car has bucket seats from a 1990 Mitsu Eclipse (don't laugh, they actually look nice and are very comfortable and were real cheap) and the headrests are a unique design that slide up and down the seat rather than extend out on rods like others do so I can't just run the straps through them. Does anyone here have any ideas?