Saw a big gas 6.4 (truck engine, but has Hellcat heads, block, forged pistons) on EBay for 4500 ish (new crate engine). Add an SRT cam, springs, Arrow intake and you over 500 hp. Get wild with your cam, port the heads and your WELL over 600, approaching 700 if your cams burly enough and you bump compression. No need for bottom end work on that one