I put rear retractable shoulder belts on mine. Lap belts were found (by data mining real world results) to not actually be of much net benefit. In more common minor accidents they can cause more severe injuries, in major crashes they're beneficial, but those are more rare.

I pondered the setup for a while. There are two mounting points per side on the bottom for the factory lap belts. Just nothing at the top. The hat shelf area also tapers in somewhat past your shoulder. It seemed like putting the retracting reels in the normal position (near the C-pillars) would leave the belt a little too far over for comfort. Could have used a loop on the C-pillar to reposition the belt's 'dangle', but I didn't think I could make that strong enough.

So I did mine 'backwards' - I put the retracting reels in the middle, on the hat shelf, with the other end of the belt bolted straight down to the center stock lap belt mounts. Then I put the buckles on the outside edge, poking up between the top and bottom of the seat near the wheel well.

To me this also had the advantage of offering more side support in a side impact. If you're on the side that gets hit, you're going to smash up against that side of the car, belts or not. But if you're on the opposite side of the car, it's a matter of being supported by the large 'double' side of the 3-point triangle vs. the point 'single' side. Plus, when not in use, the belts retract into a nice neat single straight vertical position. I used 240 sedan belts for that.

More recently I replaced the gray lobster claw front belts with some retractables as well. Mostly because the spring broke on the driver's side lobster claw. I found some neat looking herringbone black/white pattern (very fine, almost looks gray) belts in a Land Rover Discovery and used those. Used the center buckles out of a Volvo 740 rear facing jump seat. This change was well overdue, my car is a little rusty around the bottom edge (ahem), and when I tried to unbolt the passenger side low mount on the original belt, the bolt just twisted right on out of the rusty sheet metal. That's not good. :(
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)