Tydeorem,
I have a 67 F/B, so I have researched this extensively. There are two ways to do this in a COUPE or Convertible. One is to mount the belt behind the lower interior panel just behind the door. This means the shoulder belt will be anchored below your shoulder, so it must first go over the seat back and your shoulder before it connects at the lap. The second way (and ONLY way if you have a fastback) is to remove the rear corner of the headliner and weld an anchor point for the shoulder belt into the roof support, then replace the headliner and attach the belt to that support. IMHO, that's the way to go---the forces, in the event of an accident, will be better absorbed that way. However, if you're in my situation,and the car's already done and painted, it's a lot harder. Taking down the headliner, welding the support and replacing the headliner makes the chances of damaging the car much more likely. If you do this ,make sure the the welder knows what they're doing so there's no damage to the exterior paint or interior, and be REAL careful with the headliner when re-installing it.
If I can help further, just e-mail or PM me.
Good Luck,
Jay67fast
Hi! dne' here. During my restoration of my '67 coupe, I decided the install 3 point seat belts, front and rear. Obviously the headliner would have to come down to access the header where you would place/weld in the anchor, and one down below where the recoil portion goes. After doing it, I know I could do it much better now~ not that I didn't do a good job, we just learn as we go along don't we! If you'd like further info: Anocsunamoon@aol.com
After thinking a little bit(I do that fairly often!), Now, my headliner was down when I bought my rolling chassis, but I wonder if a headliner may be partially taken down? If one is careful, and mask EVERYTHING, pull the trim off, and meticulously see how the headliner may seperate from the header, but if yours is 40 years old, it more than likely won't, it would probably just rip! I installed my headliner recently(last week), and I wouldn't want to do it again! I also have my front and rear windshields out, so for me the headliner was not too bad and I did it alone. I'm full of information~ just ask!
I'll have them add an 8 inch extended door post fitting (it's an option when you go through the drop downs to build your belts) this will put the belt pull at the proper height just above my shoulder. You don't want pulling force from below your shoulder height as it could compress your spine in an accident. Look at some of the info on simpson, etc. seatbelt sites, they explain it well.
I'll be ordering the retractable with chrome buckels as they match the original interior best for me instead of the plastic ones. Chrome Lift Lever
They have lots of options and you can get nonretractable if you're on a budget. For the retractable you'll probably have to move the lower retractor back to the corner to clear your seat sliding, I will as I've installed foxbody seats.
Haven't done mine yet, when I do I'll take lots of pics.
Jon
__________________
'67 Coupe 289 C4, Candyapple red w/red deluxe int., PS, ext. decor group, light group, foxbody seats.
Next to install> stroked 333, AOD, 9" w/3.50:1, PDB.
'09 Black Warriors In Pink with glass roof - the wifes Mustang.
"If it ain't broke, I haven't fixed it yet" -Jon
I found a photo of before my headliner installation. This photo also shows a little bit of the rear seat 3 point seat belt install as well. I have more to do obviously, but maybe this shows you a little better view. Again, I would do it much better,but don't plan to do another restoration in this lifetime. I actually followed Jonks link on how to do the install. For opening of the hole in the top, I used a Dremel with a small cutoff wheel for precise cutting, amazing tool for sure! I made the nut/washer retainer to go where you see it, held it in place momentarily with a scrib(the 90degree side) by pulling, then tacked it with my mig, the finished the weld. Be sure to tap the hole back out to remove any welding slag that may have gotten into the threads. the thought also crosses my mind~ if you're involved in a severe accident, you'll probably be maimed anyway, but for little fender benders, at least will keep your head out of the window!
I had one 67 couple that had the upper seat belt mount in there already. My current 67 fb just had the place for a clip nut. I used a weld in anchor but I also used it to install this. http://www.supermotors.net/getfile/7...e/cimg0580.jpg
I think mustangs should have roll bars.. Im just funny that way I guess. But they do make a great place to secure the 3/4 point harnesses.
1967 shoulder seat belt 
), Now, my headliner was down when I bought my rolling chassis, but I wonder if a headliner may be partially taken down? If one is careful, and mask EVERYTHING, pull the trim off, and meticulously see how the headliner may seperate from the header, but if yours is 40 years old, it more than likely won't, it would probably just rip! I installed my headliner recently(last week), and I wouldn't want to do it again! I also have my front and rear windshields out, so for me the headliner was not too bad and I did it alone. I'm full of information~ just ask! 
