Hey there. Welcome to classicbroncos! And another '66 as well? Very cool.
There are several '66 experts here, so you should have an answer soon enough. There are even threads discussing that, though searching might take longer than waiting for an answer!
Just for myself though (not an expert by any means) could the code be something with a letter instead? Such as "J8" or similar?
Might be an "18" as you said, but I've seen a lot of codes with letters as well. Many of the calculators and lists are incomplete, but those few I mentioned do have other sources of information.
Good luck.
And post pics when you get a chance too. We like pics...
Paul