Not sure how much help here. Some have an engine serial # on top of the block near the crankcase breather pipe (front left side)
1972 seems to be the last year for the 170 cu.in. and 1975 was the first year for electronic ign.
Straight from the book, here is some very limited info for engine identification:
1964-1970 The basic engine in most intermediate cars is the six cylinder. in 1962 it was available in a 170 cu.in. and a 200 cu.in. size, each with four main bearings.
In 1965 the 200 cu.in. gained three additional main bearings, and the two engines have changed little since. Both are of simple conventional design and have intake manifolds that are integral with the cylinder head. In 1969, a long stroke version, the 250 cu.in., appeared. Other than its greater power there is little to distinguish this engine from its predecessors.
Maybe someone else can tell you how to tell the difference.