Many of the Zone cars were Blue toppers and even those were L-48's.
https://www.yenko.net/forum/showthread.php?t=164157
The only documentation that would match the build date of this unit is page 23 of the activity book that calls out the cars to be assigned through the Zone to the Chevrolet HQ Room at the track.
"Remaining Brass hat cars will be a zone prerogative and all requests for vehicles of this type will be referred to the Zone office"
Big block cars at the track in any circumstance (with the exception of #90 and #92) are a small minority of the total cars provided by Chevrolet.