i'm a noob to restoring to but thanks to the guys on this site i've been able to get through things i never thought i could tackle. The first thing you need to do though before you work on replacing those shoulder belts is POST PICS!! :yes:
Welcome to the site, you ever have a question post it up and sit back, lots of knowledgeable guys here who can give you the answer your looking for.
As far as seat belts are concerned, someone posted this website a while back fpor using aftermarket replacement belts..haven't browsed it in a while so i dunno how much use it'll be to you but if you run a search in the "body and interior" section for seat belts im sure you can find tons of info there. Good luck and dont forget to post those pics!
http://www.andoauto.com/hardware.htm
**EDIT**
Took my own advice and ran a seach for you in the body and interior section for seat belts and found out about Morris classic Concepts, here;s the link to their site
http://www.morrisclassicconcepts.com/