First: you're going to want to figure out exactly what the end game is here. Building a stock motor will not yield the same advice as building a twin hair-dryer big block.

Next:
If you don't already have a copy of Don Taylor's "How to Rebuild Big Block Mopars" book. go get one now

Assuming a stock (non-turbo) rebuild:

Yes, that is a forged crank

Yes, you'll want to balance the whole assembly

Stock 383 flex-plate should work and can be found cheaply.

The summit larger cam is generally spoken of as a pretty good stick for a street application.

Compression is what you'll want to focus on to wake up the 400. Milling the heads and / or decking the block will be in order to do that. You'll have to CC the heads in order to find out what they are now and where how far you'll want to cut them. keep in mind that cutting them enough to effect the compression will likely mean you'll have to also cut the intake to fit.

It won't take much work on those heads to get into the price range of a set of Stealth heads. And those would address the compression issue and offer a ton more potential.... just a thought.

BTW: I wouldn't mess with adjustable rockers. Just get new pushrods at the correct length. not too tought to figure out pushrod length, much cheaper and simpler. Adjustable rockers are not designed to make up for milling, and likely wouldn't fit correctly with the standard length pushrods anyway.

Again, if you want to keep playing with the turbo.... NONE OF THE ABOVE APPLIES.