Interesting car to model, and looks like it will require a lot of custom building. I have never seen a P68 or a PRR clearance car modeled, but after looking at pictures, my suggestion would be to acquire any 15" or 18" baggage car with 4-wheel trucks, as you choose, and start the modification process. Cutting out and rebuilding the "whisker" part will be quite a challenge.
I couldn't find plans for the car you wish to model, but plans for a P68 (the only one I could locate) seem to be obtainable from the Pennsylvania Historical Museum Commission for a fee. At least then you will know where to start.