ohhh... I assumed that you were having knee pain too.

Don't buy new crankarms. Get to the bottom of the cause of your neck and back pain first. That's unlikely to be the cause of the upper body pain. If, when you are in the proper position to fix that (by moving your saddle or whatever), you start developing pain in other areas or lost a lot of power or get dead spots in your pedal stroke, then maybe it's time to investigate crank length.

If you're sitting really far back, then I'm thinking not only does your saddle need to move but so do your bars. Your bars may in fact be too high or too close to you, because by moving your saddle back, you have 1) put your feet in front of you, which helps with your balance, but 2) increased your reach to the bars by a lot. Maybe you're more hunched up than you think when your saddle is in the right place so that you're putting too much weight on your hands rather than being in a position where you are actually going to fall on your face without the bars.

So, maybe it's time for a lower or longer stem. I don't know, but your fitter should be able to help. The best part is that they are going to guarantee their work so you can go back there if you keep having problems instead of being back to square 1 and out the cash.