My '51 actually had remnants of undercoating under the cab floor. If there was any on the fenders, that was long gone.
I used spray-on undercoating. A couple of rattle cans (I believe it was UPOL brand) took care of the bottom of the cab, and a couple more cans for the fenders. My rear fenders were lumpy from driving around with loose snow chains (it was a fleet truck, so the driver didn't care about that.)
