If you use LA rods you will need to narrow the small end. The magnum rod is not as wide at the small end as the LA so the pistons have a narrower gap in them.
Magnum is the late model...lol. Like DGC said. Magnums run a narrower rod. So and LA piston could go in a Magnum with no ill effects. But a Magnum piston either has to be widened or the rods have to be narrowed so they fit.