Seat Belts

Background
There actually was nothing wrong with the seat belts in the car.  The seat belts were the original lap belts and they were in perfectly good condition.  However, my wife wasn't comfortable riding around in a 35 year old car with just lap belts.

When the car was sold in 1975, there was an upgrade available to get shoulder belts added to the lap belts.  There is a small knob on the buckle of the lap belt that allows the shoulder belt to be clipped in.  Now, if you're lucky enough to locate one of the original shoulder belts at a good price, definitely go with it because install is a snap.  Retrofit belts from a Corvette catalog easily cost hundreds of dollars.  For those not wanting to spend that kind of money, there is another option.

The rear seat belts from a 1993-1998 Camaro are known to fit perfectly in most C3 Corvettes.  I was able to get both belts for about $85 shipped to me next day from a junk yard a few hours away (didn't have time to drive down and get them).  Some early C3 models require a metal bracket to install these seat belts because the bolt hole is in a different position.  Here is a link to a forum containing information on the bracket as well as pictures from others that have made this modification:
[Corvette Forum] camaro seat belts in a c3, is this true?

Considerations
There are a few things you need to check out before switching to shoulder belts.

• Are the inside rear quarter panels meant for shoulder belts?  There are two different panels, one with an opening for the shoulder belts and one that's just a solid piece.  Solid piece panels won't work.  I'd be very careful if you decide to cut the hole yourself cause those panels are something like $300 to replace.
• Do you want the seats to have feed-through?  The original option changed the seats to have an opening to allow the shoulder belt to pass through and it would keep the belt in the correct place.  If the seats don't have the guide, it's not really an issue.  The shoulder belts pass easily on the side of the seats, but they will fall behind the seats when you leave them.
• Are you fine with a non-stock look?  The Camaro seat belts are not ugly or cheap looking.  But they definitely do not look like they came from the 1970's.  At least the ones I got have a red button instead of black/chrome.

This is the drivers side belt installed.  I've removed the quarter trim panel to make it easier to see.

Procedure

1. Start with removing the seats.  There are four bolts (one at each corner).  To reach the back ones, you'll have to push the seat forward as far as it goes.
2. Remove the door sills so that the carpet can be removed/lifted later.  There are small black screws holding them in.
3. In some cases, the rear quarter trim panel and retainers hold in carpet, removing them may be necessary.  If there's no hole in the carpet for the shoulder harness, then you'll have to remove the rear carpet to find the hole and make it in the carpet too.
   
1. Remove the rear quarter trim panel retainers.  These run along the inside of the door jamb.  There are a number of screws holding them on, once removed the retainers sort of slide off.
2. With the retainers removed, there should just be 1 or 2 screws holding the quarter trim panel in place.
3. I don't really have a good method for making the hole in the carpet, but I used a sharpie and my arms to get the holes position.
4. Remove the original seatbelt.

1. A single bolt holds the belt buckle to the center console.  Some models have a seat belt sensor (like the '75), you'll have to remove the center armrest to get under the carpet to disconnect them.
2. Under the carpet, there are two bolts holding a guide bar for the seat belt place.  You don't have to remove them completely, but just enough to allow the end to be free.
3. Also under the carpet, there is a single bolt holding the wound belt in place.
5. Now it's time to install the Camaro seat belts.

1. I would suggest using the plastic buckle covers that were on the original buckles.  It's a huge pain getting those off the old and on the new buckles, but if you don't use them, the buckets tend to fall between the center console and seat.
2. You should be able to use the original bolt to secure the buckle.
3. The end of the Camaro seat belt has the wrong angle on it.  You'll need to flatten it with a vise or large hammer.
4. Bolt the seat belt to the hole in the rear compartment that is behind the rear quarter panel.
5. Pass the seat belt through the rear quarter trim panel, through the seat if you have the pass-through, through the hole in the carpet, behind the guide, and loosely bolt the belt end to the floor.
6. Double check to make sure the belt is not going to be twisted when you put it on.
7. Tighten the belt guide and belt end down.
6. Re-install the removed hardware
1. Rear quarter trim panel
2. Rear quarter trim panel retainers
3. Center console if you had to remove the seat belt sensors
4. Seat
5. Door sillf