I have read and seen people do it both ways, I usually install the pipes before shaping it down for some reason that I couldn't really explain. In reading through the post I am pretty sure you are only speaking of the forward entry thimbles, but just in case you are also refering to the rear entry thimble, always inlet that before shaping it down, I have a unfinished rifle just sitting in my basement because I tried to do it the other way:(