Ray, if the non-air cab tractor is the one you have on construction, I would strongly recommend putting air under the cab.  I know Kenworths are supposed to be immune to cab problems but I can assure you that 8 years of  off road use on two of those inferior brand trucks we run shows a tremendous return to the air ride cab on one compared to the other without, and the cabs are otherwise identical.  

Neighbors have put air ride on several truck cabs they run, I don't think it was either expensive or difficult.