Stadard Bushmaster stock is A2 length, the 'fixed' collapsible is A1 length (5/8" shorter). Since the BM 'fixed' collapsible uses a full length buffer tube there is NO way to shorten it (Bummer I know).
You have 2 options:
1) Go to a 'parts' company like Model 1 Sales or something and ask them to 'fix' a stock at a particular length. I'm not sure of the actual legality of the doing that (If its BATF approved) I doubt it, but they do do it.
2) Go to Rock River Arms website, the offer a short fixed stock (shorter than A1) for 'entry teams' IIRC. This might work (I have recently learned this stock is 7" long - hope this helps).
Hope this helps.