I am running a new set of rear Alcan springs and they are great so far. I chose them over Marlin springs because I didn't want to move the perch and shackle (the cost adds up to about the same in total) because the stock length springs don't allow as much body roll (less flex) and I drive mine on the street and highway often.
If you are looking for max flex, Marlin is probably a better deal (the springs are built for a weight between a truck and runner so they are a bit stiff on the trucks and soft on the runner) because they are a great spring at a lower price. My springs were $190 each and I think the longer ones are about $20 more, plus you would have to pay or do the mount/shackle mods. If you have the cash, Alcan can build a far superior spring, but it isn't always worth the money. I found on mine that my shocks are actually the limiting factor until I relocate the mounts. If you don't want to do a bunch of fab work, or pay for it, Alcan is a great way to go.
