You just need a self adjuster kit for the same size shoes.
68 C10 self adjuster kits from Napa
https://www.napaonline.com/en/search..._-_pickup/1968
16.33 a wheel here if you don't get a buddy deal discount.
to do it correctly I'd invest in new hold down springs that matched while I was at it
https://www.napaonline.com/en/search..._-_pickup/1968
That isn't going to leave you any beer money out of a hundred dollar bill but you will have self adjusting brakes.
Those are for 11 inch shoes. The only thing that might not jive is if you still have old enough shoes so the hole on the secondary shoe is too small for the base for the retaining spring to fit in.
This isn't rocket science with some exotic parts it is just an update parts change.
Expect to order and wait for the parts to come in on a stock order. That should save any special shipping costs.