I was once told to leave the 400 alone--a 350 would run just as well. Lucky for me I didn't listen. With no more than flattops, stock #291 heads, a hydraulic cam, headers, an ancient Weiand manifold, a Turbo 350 and 3.55 gears, my '68 ran mid-12s. It cost me about $1200 to build that engine--including machine work.
The 400 is a great engine, but you MUST keep it cool! If it overheats more than a couple of times with forged pistons and decent compression, expect the #7 cylinder wall to crack. A 400 will take a .030" overbore with no problems as long as it's kept cool. You don't need 5.7" rods--the stock rods with ARP bolts will easily live at 6500 rpm at the drag strip--but a 5.7" rod cuts down in internal friction. The stock iron crank will live at 6500, too. Even if it's a 2-bolt, you don't need splayed main caps unless you spin it well over 7000 rpm and have very high compression--that would be a waste of money. Any heads with 200cc intake ports will really wake a 400 up if you're spinniing it to 6500. But remember to DRILL THE STEAM HOLES!!! It will overheat if you don't.
There is no substitute for cubic inches, and you can build a 400 as cheaply as a 350--and it'll have a lot better torque! For that it's worth, that's my input and good luck!