Flexplate is not fits the torque converter

Magnums have 10-bolt valve covers. The Magnum intake manifold-to-head bolts point straight down. LA has the intake bolts perpendicular to the head. If you pull a valve cover, you'll find pedestal-mounted rocker arms on a Magnum instead of LA shaft-mounted rocker arms. If you remove the intake manifold you'll find roller lifters on the Magnum. Front end parts like water pump, alternator, p.s. pump, had serpentine belts on the Magnum (however LA parts also fit). I'm sure the serial numbers stamped in the block would tell you something, but I don't know what you'd look for.