Good info as I'll need to be doing something about the belts for the '64 GTO I am trying to finish up.
I can't offer any specific recommendations but I can say you need to be careful. Certain dyes or dying processes can weaken the belt webbing material. You would probably be better off replacing the webbing with new.I should dye my belts black again, last time didn't come out too great. Any suggestions on a certain black dye to use?
Something that needs to be considered. However, earlier this year I decided to refinish the black seat belts in my '68 El Camino myself. I took the plastic buckles off and refinished them. I soaked the webbing in warm water with Simple Green. They were dirty! After they dried, I "painted" them with Spray Max 1K black satin trim paint: http://www.66autocolor.com/Spray_Max_Trim_Paint_36801_p/spm-36801.htm . When I went to my local auto paint supply store, I picked up a spray can of the equivalent product by SEM. The clerk said I should try the Spray Max product instead, so I did. Worked great!