Looks like our Tuneup Specs need correction. Those 68 350 numbers are carried over from a 67 430. Shop manual gives a 45TS plug and 0 deg TDC timing for the 68 350. Also in there is a description of the Controlled Combustion System. A blurb describes how emissions are reduced by retarding the idle advance to TDC.
If you could post the part number of the distributor we could tell more about what we're working with.