Engine and trans

[SleepO]

So I had a quick question about pairing up an engine and trans. I'm not a mas
ter mechanic but I can find my way around an engine. Quick background on my vehicle; it's a 64 lwb with a 230 and Muncie 3 speed transmission but converting it to a short bed. I am looking to swap the engine with a 350 and transmission to an auto th350. Is this a good matchup? If so, do I have to swap the gears in the rear end? Am I going to need the short tail shaft or long tail shaft? Do I need to move my cross member? This is where my limited mechanic skills hold me back, I'm not very good when it comes to transmissions and gear ratios.