The big Kookas are made from a 7000 series aluminum alloy. It's a harder and stronger alloy than most other aluminum cranks. Mix that with the big beefy design and they're stiff, strong and solid. The harder alloy means the taper and pedal threads hold up well.
Unfortunately Kooka is out of business. The only way to get the cranks now is to find them used or get really lucky and find a place that lost them in storage and is now selling them as new old stock.
'daVinci Designs' (http://www.teamspirit.net/comp.html#cranks) makes an adjustable crank set that has pedal holes for 130mm, 150mm and 170mm. They're expensive, but they have the advantage of being available for sale.