Welcome to the site.
Since it seems like you don't have a lot of experience with these cars let me offer some information. Reference your questions; as Ian said the exhaust is the same. Reference the clutch, it depends on what you engine you put into the car, if it is relatively stock then pretty much any appropriate clutch will work (perhaps the one you have now).
Why do you want to go to a big block? I have 2 E bodies, a Challenger with a 440 and a 73 Cuda with a 416 stroker (stroked 340). The stroker makes really decent power (500+) but it doesn't have the weight. Don't go to a BB just because you think its expected or "better". Nothing against a big block but with today's technology and parts you can make a SB produce near or the same power levels.