Small block to big block convert questions..

I've been bouncing around building a monster small block for my streeter. But I'm having questions on the reliability of that build , so I'm kinda kicking around the idea of building a big block for the 73 dart.
Question is what are some of the big issues in making a big block fit in the small area under the hood? I know the drag guys dump the fender wells and etc to make room, but since this is a streeter I need to keep those.