I have the All Pro 6" rears (same mfg as marlin supposedly) and they are REALLY soft. I have a 1st gen runner and they flex really good even without the top on, and 1st gens are a lot lighter than 2nd gens. If the bottoming is the only problem, you can just get some progressive bumpstops to mellow that out. How is your shackle angle? If it is 45* at ride height, you could change it to more vertical to stiffen the spring up.
