I'd guess the truck has had more than one owner over the years.. even if somebody had done the upgrade in the early 70's that still would have been 35 years ago.
In fact, i'm suprised MORE of the trucks haven't gotten that upgrade.
Unless the guy you bought it from was a well read stovebolt nut, I doubt most folks would know that it should have had a torque tube driveshaft.
Unless your going for a 100% original resto, which I assume your not since you were asking about lowering it, i'd consider it a blessing.. since a upgrade from torque tube probably means better rear end gearing for higher top speed.
