Ha! I'm putting a Gen VI in my 71 right now.
The big hassles are:
1) Oil pan. You basically have to buy a new GM, Milodon, Moroso oil pan. As there are no factory "cars" with this engine only trucks. And their oil pans don't work.
2) Also, the flywheel is different somehow. I happened to have an extra one, but they aren't the same as a MkIV. I'm not sure about the flexplates.
There are some other differences.. if you are getting one from a truck, make sure it has a mechanical fuel pump pad, and a drilled and tapped spot over the oil filter for the clutch pivot if you are going to use a stick trans.
You can use normal intake manifolds, so you can use whatever you want. Same for headers.
There are other internal differences, like the cam and stuff. That's the big advantage. Factory roller cams. Cheap. In fact, heres my ad for my old setup. (sorry for the selfish advertisement, but it drives home the point)
http://sacramento.craigslist.org/pts/1241364941.html
I mean, where else can you get a full roller valvetrain for $165?
That's where a Gen IV shines over a MKIV.
