I had my Westclox/General Time clock converted to quartz a few years ago and it keeps perfect time. This is the outfit I used:
Clock Works
1745 Meta Lakes Rd.
Eagle River, WI 54521
Phone: 715-479-5759
WWW.CLOCKWKS.COM
The website indicated that they had some 1968 Chevrolet clocks in their inventory when I had mine done.

I hope this helps.

Mike