Small block differences per application

Hi, I am looking to replace the /6 in my valiant with a small block V8, and I really haven't decided to go one way or the other, but I think I want to buy something that is complete and running , then rebuild it. This is going to be more of a budget build but want it to be reliable.

With that said, I want to buy a small block 318/340/360, and I have seen engines out there that come from RV's and trucks that are either 318 w 727 or 360 w 727.

So, what is the difference in these engines when built for an RV or truck vs a car?

Please pardon my ignorance, I'm a GM convert and trying to stay away from putting an LS motor in my mopar.