I had 4wheelparts (Tacoma, WA) install my 4.5 inch Superlift a couple of years ago. The only problem that I had was they destroyed my rear shackles when they took them off and I ended up with extended shackles that made the truck way to high and caused vibration. I had to find some stock shackles and install them myself. I currently have Alcan rear springs on order, the Superlift rear springs are terribly rigid.
The Marlin springs are longer than stock and will require you to move the foward mount and the shackle mount, the Superlift springs wouldn't require this. I opted for an Alcan spring that uses the stock locations to avoid the added install problems. The cost is similar when you consider purchasing mounts and fabrication work. I'm sure the Marlin springs are great quality if you are willing to do the work.
Something to consider.
