Sounds like you are looking at one-piece shower piping units - you can assemble this piece-by-piece ones so the tailpiece (pipe from the drain to the trap) length can be changed at will to get drain matchup.
If you find a one-piece that fits OK except for the shower head, you can get chromed and brass and plastic shower arm extension pieces to extend the shower arm if it will show - or use an appropriate corrosion-compatible material pipe nipple (depending on what material the showerhead and the plumbing pipe is made of, might be galvanized steel or brass or rarely stainless), to adapt between the drop ear elbow on the pipe in the wall (looks like this) -
http://www.leemason.com/wp-content/up...
and the shower arm (which is usually chromed). Generally speaking, you are better off removing the shower arm (if it comes out easily - don't reef on it and possibly break the pipe inside the wall) and putting in a new longer one if needed to be able to reach the drop ear elbow. If you just want to extend it on the shower side of the surround, you can remove the shower head and buy both straight and curved and swivel shower arm extension pieces to go in there - though that is generally going to lower the shower head height from the floor too.
If you mean with the unitized assembly you have to push the shower head pipe BACK into the wall further, going to be tougher because it will run into the drop ear elbow, which would (along with the stud or the crosspiece of wood it is mounted on) have to be pushed back too, if there is room.
Personally, except for "stall" type installation where there is free room to play like with some beachsidse and poolside type showers, I don't like the unitized ones - just build it up from parts to fit.
