Im not certain but I think LA roller lifters are a different set up because of the lifter bosses on the magnum were designed for a roller whereas the LA engines werent. I was at one point wanting to buy a retrofit roller LA for the magnum but was told that the lifters were a different setup and would not work. If I remember correctly the retro fit lifters have a bar connecting two lifters at a time together which would interfere with the taller lifter boss on the magnum. As far as a retro fit LA roller cam only with used with stock magnum lifters??????