The rules specify the cage tubing based on weight of the car. Not spec line weight.
[/b]
Quite the contrary, copy/pasted from the GCR:



D. For purposes of determining tubing sizes, the vehicle weight
is as raced without fuel and driver. The minus tolerance
for wall thickness should not be less than .010” below the
nominal thickness. Improved Touring roll cage tubing sizes are to be calculated
based on the published vehicle weight minus 180 lbs."