RBs fit fine, with the right engine mounts. There was a very old article from Mopar Action (think early-mid 1990s, could be off on that) that had an engine swap article that put an RB into a Mirada. Everything was in place: power steering, power brakes, Air conditioning, cruise control. Stock hood, no scoops or carbs sticking through the hood. Weight difference was low enough that only the ride height adjuster needed to be tweaked. I think if this were done today, using a low rise aluminum intake manifold, aluminum heads, and moving the battery to the trunk would address at least half of the weight increase between the small block and the big block.
Hemis are heavier and wider than the RBs, so I don't know about the weight or fit issues, and I don't recall seeing any swap articles. Doesn't mean that is isnt' possible though, or that somebody hasn't already done it.