I kind of have the same plan. Is it necessary to upgrade springs with a mild cam upgrade? Rookie questions but what would be the maximum cam lift?? using stock springs in a 5.9 magnum?
Stock valve springs are pretty weak. An upgraded cam on the stock springs will be ok at lower RPM but will get into valve float easily. Not really worth it.
Max lift on the Magnum with stock pistons usually comes down to the clearance between the retainer and guide, or more realistically the retainer and the seal. Somewhere around .540" lift the Hughes spring usually clears, but each one is a little different.