Overdrive tranny in an A-body

Supershafts said
Not to be a sour grape, but why the mag motor....

I would have to say why not?

A stock 5.2l magnum motor makes a bunch more power than a stock 318. I you plan to modify it the magnum 5.2 already has the best small block heads mopar delivered from the factory and it is already set up for a roller cam.