Stupid question but . . . . why are you changing the plugs ?? Were the old ones working ok ??? If so then just put the same thing back in and keep on driving. If you cannot physically get a 13/16 plug socket in the hole then you have already answered your own question.

The head casting number is probably under the valve cover.

 

And, you can do a really decent job of cleaning spark plugs with gasoline, PB Blaster, etc. then wire brush them. Use sandpaper to clean the electrodes.

 

Bill is correct, the casting # will probably be under the valve cover.

As far as which plug reach to use the 13/16" socket may not fit deep in the well enough to reach the hex head because the wrong/tapered seat plug sits too far into the head OR it could be that the head has been replaced with a newer one. The only way to know for sure is from the casting number.

 

Interesting car. About the newest that head or motor can be is mid 70's, especially the intake is original. Since chebby went to the one piece head and intake around that time. What rear gear ratio does it have? A straight 6 with the 2 spd must have leisurely acceleration.

 

Thanks. Yea, its not fast off the line but the passing gear really kicks, when you need it. Its a 2 speed.