1969 barracuda big block conversion

I have found a nice 1969 Barracuda coupe at a reasonable price, but it is an automatic 6 cylinder. I have several engines available, including a good 383 and a good 440, so my question is this. What do I have to do to convert the 6 cylinder to the big block engiine with auto? I want the installation to look stock, so I also need advice on which engine to use. Should I use the 383 or the 440? Is one easier to install than the other? Any advice would be greatly appreciated.