what's up with the 5/8 plugs? this is the confusing part. My guess Heads from some other car 75' and up maybe. Thank You, Bill

The block casting number isn't going to tell you its for a Shelby or anything else really.  Shelbys didn't ever get specific engines.  The early cars that Shelby actually touched showed up at his shop as 289HiPo engines and some of the bolt ons were changed to increase power output.  The amount of actual Shelby in the cars declined from the first year to the end of production.  By the end they were mostly loaded up cars with an appearance package using the Shelby name to sell more expensive cars for Ford. 

The block casting number simply corresponds to a '69-'70 model year 351W.  Could it have been used in Shelbys?  Sure, but it was for obvious reasons more likely to have come from something less valuable. 

As for the partial VIN, unfortunately it doesn't prove anything.  I'd love to tell you that no one ever stamped a new VIN onto a milled down pad (removing the real number and replacing it with another), but it happens all the time.  Sadly, when money is involved people often do unscrupulous things.  The correct heads should have the larger plugs as noted, as the early 351s used the larger 18mm plugs with the 13/16 hex. 

Nos681 wrote:

If you pull valve covers off, there will be cast marks in head to help you determine year and size motor heads were made for. 351W C9 heads use 13/16” spark plugs.

My '70 D0OE heads use 13/16" plugs.  I would imagine '69's would use the same plug, and the next few years, too.
 

Thank You Everybody...I got it now.  I have a 69' Mustang 351W block C9OE and someone put later 1975 heads on it. explains the 5/8 socket 14MM plugs. Thank You, Bill