There will be a part number stamped on the distributor breaker plate near the points that can be cross-referenced to a fairly specific range of installation as original equipment. Just about any Chevy 6 cylinder distributor can be installed in any engine for a span of 15 -20 years of production, and vacuum lines can be altered to fit, so going by the physical characteristics of a particular distributor will tell you nothing about the origin of the engine. Casting numbers, and the serial number stamped onto the machined pad near the distributor are much more reliable ways of tracing an engine's original application.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!