Converting over, need some advice

Welcome to Mopars and FABO! Glad to have another convert from the 'Dark Side'. You're going to have fun here.
If you're up to the possible body work, the '71 Demon would be a good choice because Demons were only built in 1970 and 1971 - it became the Dart Sport after that.
But the amount of bodywork necessary will be the deciding factor in that purchase.
Contrary to what was stated earlier, I believe there is a great deal of interchange between Mopar engine families - both smallblock and bigblock. I happen to like the smallblocks better in A-bodies as I believe they give a much better balanced package.
Good luck on whatever you choose and show us lots of pics of your purchase and your build progress.