At $1,000 for the adjustable power cranks ($1200 if you want a lockout to be able to use them like normal cranks) it would be more affordable to have a machinist custom fabricate a left side crank of the proper length... assuming the proper length is known.
If it is not known, then an adjustable crank would be the only way to ensure that buying one item gives the solution... But at the same cost as several attempts at cheaper solutions (unless the OP is using high end equipment).
The OP could shop around and buy a full middle range drivetrain group with a shorter crank length just to use the left crank and throw the rest away and still be less expensive than the powercrank. If the OP does this, please tell me where you are throwing the remaining components.
If he is riding a Trek 1000, it would be cheaper to buy another Trek 1000, get the cranks swapped by the LBS (for little or no cost) and then pull off the left crank for his current bike, and throw the rest of the bike away...
If there is no good information on how much shorter the crank needs to be, I would just go for buying a couple of left side cranks instead of full groups or bikes. If 168.2 is the proper size, then custom work or the adjustable cranks are the solutions that would allow that.
If someone is looking for a different length crank, I suggest that they just buy one... However, if someone also wants the features of the Powercrank, or adjustable length then my whole argument collapses... and I suggest that you buy one.
donrhummy, What crank components are on your bike now?