Replacing sheetrocked/tiled walls with fiberglass unit


I have just ripped out my old tub with tile walls (tiles were falling off due to lack of greenboard) and have put in the tub part of my new 2-piece fiberglass shower unit. Now I notice that the existing manifold and valve stems do not extend the extra 1-1/2" to where my new walls are. The fiberglass walls are 2" away from the studs, and my valve stems reach 2-1/2", so there's no room for the finished (chrome) collars.
What are my options? I have tried to get replacement valve stems at local hardware stores, but no one carries the Kohler fine threaded valve bodies to screw into my manifold. The only options I see now are to replace the manifold (ick!) or extend it by cutting it out and piping it up and out to meet up with my new walls.
Help?
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I just got back from Lowe's across town and the guy there suggested one of two things:
1) Get a longer stem or a stem extension from a Kohler supply store or 2) Get a shorter collar from a Kohler supply store (I forget what they're really called...the chrome flared piece that is screwed onto the stem via a white plastic adjustable sleeve)
Is there anything I should be aware of here, or is this sound advise?
With that long of a piece extending from my manifold and mounting onto fiberglass, should I be providing any other support?
Thanks for any suggestions.
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I've given up and bought a new single valve and will be sweating pipe this weekend ;-) I got tired of running around town and the local supply store gave me a valve for ~$70, which was good enough for me. Now I'll have a new shiny handle for my new bath tub, anyway.
Thanks for your time.
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