Where are you having pain?

I think all of us just assumed you were having knee pain because of your wanting shorter cranks... but even if that's the case, there are a lot of kinds of knee pain. Some call for shorter crankarms and some don't. As you've found, cranks are about the most expensive component that you would replace for fit reasons alone, and it definitely isn't the first thing I'd try if I wasn't sure where my problems were coming from.

Have you read through the threads about fitting in the Health section? I personally haven't read Andy Pruitt's Complete Medical Guide for Cyclists, but a lot of people here swear by that book. It would probably be a good place to start with fit questions.

And honestly... if your LBS was telling you to replace your chain every 500 miles without measuring it... OR if it really was worn to the point of needing replacement after 500 miles and they never helped you investigate why... I don't know if they'd be the ones I'd trust with fitting, either. Unless possibly you misunderstood their advice and they were telling you to take it off for cleaning??? (I don't do that either but some do...)