318 magnum VS 318 roller VS older 318 gas mileage

I can't speak to the auto tranny questions but it sounds like you've already got the info you need in the last couple of posts.

I also put in a new timing chain. I believe I used the harmonic balancer off the Magnum however as my old one was not great

The entire front end of my old 273 bolted right on to the Magnum. Alternator brackets, timing case, water pump - Even the fuel pump. You'll need a cam snout extender for the pump. Hughes Racing is where I got mine.