Z06 wheels weigh more than 18 I believe and think over 19 (I'll weigh one this weekend) so shoudln't be because of that. Sounds like it was for the Mustang and the Enkie wheel? I'm not aware of ever seeing an F body with a welded wheel. A director probably got a good deal on some lightweight 17" CCW's or something and wanted them legal at the time for the weight. Raise the weight to 19 lbs without a welded spacer.
The logic in a budget series to allow buying a $2k set of wheels and then welding on a spacer just so it's legal doesn't make sense to me.