Rick
check out
http://www.powerpacspoilers.com/c337.htm
This is the only system that is approved (STC'D) for the 337. Precise flight is the other manufacturer but does not have a product for the 337.
Takes about 50 hrs to install and somewhat more if it is pressurized. The powerpack sits on the rear shelf of the rear engine .. on the pax side. Hydraulic hoses run from the power pack to each wing rotory actuator. Rather simple plumbing.
Powerpack has two flat panels that pop-up which will provide about 1000 ft/min rate of decent without changing power (at the yellow line).
It is great when you need to slow down quick or increase the rate of decent above 1000 ft/min without shock cooling the engines.
I use then ALL the time especially when ATC asked you to keep your speed up on a final approach.
Sure saves sticking your arms out the window....
Bob