Big Block or Small Block in my 70 Duster
Last summer I did a 400 into a /6 '71 Duster. It required a swap kit from
http://www.engine-swaps.com/ that made life easy. They didn't have at the time the better fail safe mounts so I purchased the torque strap. That required a drilling of one hole.
Next are headers. Exhaust manifolds are possible to use IF you can get the proper units.
Being that money was tight I went to the tech pages over at Big Block .com and looked up the headers I had on hand, used out of my '79 B body car and just simply followed instructions.
http://www.bigblockdart.com/techpages/techindex.shtml
Next is a drive shaft shortening to fit to the upgraded rear swap I did. The /6 was a 7-1/4. I went to the 8-3/4. The distance is a bit shorter then stock.
The next required parts you'll need/want are a better radiator and brakes.