360 Question

Well, if $600 or so gets you the whole van, it sounds like a pretty good deal to me. You get the 360 with the transmission for your Dart, and sell parts off the rest of the van to make back some of your investment. Win/win.
The only issues to putting that 360 into the Dart are basic- it will need a 360 passenger car oil pan and pickup, 360 engine mount brackets (the 318 ones will work too, with a couple of quick modifications- lots of threads on here about that, it's no problem), and the need to address the engine balance difference: the 360 is externally balanced, the 318 is internally balanced. There's a couple of ways to address this, depending on the Dart and the van transmissions. If they are both 904s, just run the torque convertor that came on the 360, or run the entire van trans in the Dart, which is a good opportunity to do a freshening and mild performance build on the trans. with the boy, too.