Hi Fritz, you probably know this. One thing with using poplar for sheathing -
When the car is primed and painted, paint the bottom edge of the wood as well. The sheathing will come down an inch or so past the bottom of the sill, be sure to paint the ends of the wood to seal it from water leaching up into the grain when the sides of the car get wet. Painting the bottom edge helps the sheathing last longer and prevents checking (spliting) from the bottom as often happens when the sheathing has been on the car for a few years.
Stewart