Go aftermarket. I did. I wanted to do a set of 74 belts in my 71. Whata a PIA. In fact RIGHT NOW I have a complete set of 74 belts UNDER MY DESK here at work. I actually have 1-1/2 sets. they are so hard to find in decent shape I bought a complete set and some parts.

Heres a write up did from a couple other previous posts...

"First, the best way to swap them would be to get a COMPLETE set of ORIGINAL 3pt belts from a 74 E body. That was the only year E body that they came in. BUT they are expensive, since rare, usually around $200 just for the belts that are crap. THEN you also need the even rarer rear interior panels that the retractable belt came up thru. PLUS I checked, it's about $600 for Snake Oyl to restore the belts! Bottomline - this way is nearly impossible.

The way I did it. Bought the 3pt retractable with the 8" dogbone (the plastic covered steel strap that lowers the shoulderbelt down from the roof attachment point - you can see it in the link of pics I'll provide below). The company is http://www.andoauto.com/retrofit.htm The ONE PROBLEM with their belts is the plastic cover on the 12" belt/buckle that mounts to the floor beside the front seats. It's a cheesy cheap plastic cover with the wrong angle. Worst case, it IS usable but this is what I did. I searched for a correct shaped cover. There are probably many applications out there that would work. Probably even some new cars. I bought a set of rusty lap belts off Ebay for a 73 Cuda. The original plastic covers, actually more of a heavy rubber, WERE PERFECT! I did pay $40 for the set but I really wanted the installation to look correct. The complete 3pt front set was like $175 from Andover Auto. Not too bad I thought. Just a bolt in. The look and work perfect."

I can post pics tonight if interested.