I have the N66 (SS) wheels, - maybe that's the issue???
Apparently it is.
In 1969, YA (N66) wheels were used on the Camaro, Chevelle and Nova. The P&A shows it as P/N 3949658. Unfortunately, I can't find any information about the length of it so the only option would be to have someone that had N66 wheels from the factory measure theirs.
Update: I did find where P/N 3949658 was superseded by P/N 1386521 in October of 1970, and even found one on
EBAY that the seller states it to be 8" long (see picture below).
Be advised though that this doesn't mean the 3949658 was that length since replacement parts are sometimes different than the originals. Also note that the 1386521 was also used on the 1970 full size passenger car (except station wagon and convertible), the 1971 Nova with 350 and the 1971 and 1972 Oldsmobile with the "Custom Trim Wheel" whatever that was.
Ed