re-doing the brake system for the 68. It was a factory drum setup converted todisc in front by previous owner. As far as I can tell, it looks like all he did was install different spindles with rotors etc. Now the calipers mounted are single piston ones. Yet the service manual only lists four piston calipers on 68's. Would these singles match up with any other year chevelle??

Not sure exactly what your question is.
A 68 4 piston set-up can be converted to single piston by;
Changing to single piston rotors, calipers, caliper brackets, dust shields, brake hoses (and frame brackets)
The spindles are the same so no need to change them....Bill Rose

69-72 uses the single piston calipers. The 68 has the 4 piston setup from the factory, but all the kits that are sold are single piston. Also, if it came from any other A body donor car 69-72, they are a direct bolt on to the 68. So if you need parts, just look up a 69.

thanks guys. Found the right ones while looking up under a 69