440 duster

This is a VERY easy swap. The trans tunnel and hood will be fine. If you use a tall carb spacer and/or a tall air filter you may need a hood scoop though. The 6 pac scoop always looks cool on a Duster.

If you are going to make or modify some motor mounts you might want to get the headers first, just to make sure they still fit right.
Schumacher makes motor mounts for putting a big block into your Duster for $150.00-180.00, here's the link http://www.engine-swaps.com/

As for headers, there's a few different ones for this swap, I like the pro-parts ones because they make good HP, and they don't have tire or ground clearence problems like some of the other ones. There a slip tube setup which makes installing them very easy, but you will have to cut some holes in each of the inner fender wells for a couple of the tubes.

Your radiator will probably need to be beefed up for better cooling and you might want to switch the inlets and outlets to the other sides if you decide to just use a stock small block one, to make routing the lower hose easier. Or you could just go with a good aftermarket aluminum one.
Good luck