When I recently install a Delta 1700 in my shower, I sweat a male adapter on a piece of pipe that I knew was a bit longer than would be necessary. Then I screwed that assembly into the spout and cut the end of the pipe off to fit. Worked great. As far as reusing copper pipe, there is no reason not to reuse it. Even fittings can be reused, however most of us don't because it just isn't worth the time and effort to clean the solder out to make them fit on the pipe. That plastic pipe is CPVC and you should know that the inside diameter is much smaller than copper pipe of the same size. It may not be perfectly accurate to say that 1/2" CPVC is the same as 3/8" copper, but I think it pretty close, and that is a huge difference pipe. Do the math to figure the area of a circle and you can see what I mean.