There is nearly exactly 100lb diff between iron small Chevy and big block... not that much as far as weight... room in engine bay will be more restrictive to do headers etc on big block, but you can get them. If you want headers, get them in there before you drop in the 454. Yes, the 4L60 can live there, but if you're going to tow, climb, heavy foot it a stoplights it's not something to count on for lengthy service. Remember, the 454 is an externally balanced engine so you'll need a big block flex plate for it with that 4L60. Get all the little balancing parts or your motor will vibrate badly! You should already have the correct damper on the front of the 454.

Yes, you'd need a 350 to build the 383.