Chris,
Thanks for the tip...I tried your method and also had some good results using the #11 blade turned around (sharp edge facing up) and slowing nicking out the material by flicking the knife up. The back of the blade rides in the any part of the old groove that was still available and keeps the new slat straight. This method allowed me the most control for getting the lines straight and not allowing the blade to drift off mark. When I slipped up and created a new odd width board, that didn't match up with the others, it just added more character this old car ;D.
The dragging blade backwards tip worked ok for removing the new plastic fuzz & flash created from the material removed. For that task I found that a brass brush or short bristled paint brush, gently swirled over the area, also worked ok. Didn't want to ruin all my previous hard work from above. Hope the final results stand up to macro photography :-\.
Now for the next phase...laying out the grab iron locations, drilling out the holes, forming them from wire & then glueing them in place. How do you pro's handle this task?