I just finished converting two GPM models back to O gauge for a buyer who did not want P48. It took only 1:45 per loco. The most time-consuming task is attaching the power reverse linkage to the driver axle. The cylinders are a little wider than the prototype but it isn't really that easy to pick up.
I did cut down a Sunset SP P-8 cylinder saddle. It took a while. I had the first one done by a friend and the second I did myself after seeing how he did it. That is more time-consuming but it is a huge improvement over the thee-rail dimensions of the width. I believe I cut about .090" total to get it to match the SP drawings.
Gene Deimling