Procharged 427" LSX ,, 800+ crank HP to get you the desired 700 at the wheels. It's becoming a pretty common FAST EFI deal. Lots of base maps out there now. But a $25-30k+ price tag by the time your done ain't uncommon either.
As much as it pains me to say this,, 671 blown 468 big block will be your cheapest / reliable route to your goal. When you start force feeding a motor you don't have to have mega buck trick heads to make the big HP numbers. Just crank up the boost to compensate for any head deficiency.
A 700+HP 383 is not a candidate for ANY production GM SBC casting,, sorry. I've dumped the $1300 into a sonic checked GM production casting with splayed mains, deck plugs, short fill, bla bla bla..... all to have the deck and bores fail. It's just not worth the gamble. Do it right the FIRST time is a lot cheaper than over and over.