Block ID, spark plug question

I had a hard time tracking down the suffix code CKB for anything corvette related and so I ended up tracking it down on these websites.
http://www.nastyz28.com/camaro/camaro72.html
http://www.novaresource.org/codes.htm
Now the reason I'm stuck on it being a 72 motor is because of how I'm reading the partial vin, wouldn't the second digit be the year in this case?
The first practical stumbling block for me in all this is trying to figure out which plugs I need to get...If I've ID'd the block correctly then I should use a 4006 bosh platinum. The existing plugs are Autolite 86's and they look to be completely different beasts... 13/16 vs. 5/8 and there is no cross-reference that matches those two plugs.
Any feedback I can get from you guys on decoding all this would be greatly appreciated.
Am I on the right track with the ID of the motor? I stopped by my chevy dealer yesterday for something else and asked if they could pull up any information on those ID's and they said they had no way of looking it up
As far as the plugs go... in general terms without worrying about specific part numbers, would you expect a 5/8" plug to substitute for a 13/16" ? The reach is the same on both plugs, the seat is completely different though.
Any ideas out there that would let me know if I'm approaching this all wrong or overlooking anything would be greatly appreciated. Thanks!



