Not hard at all to get 400hp from a 383. First question, how far down in the hole are your pistons? Next, what is your rear end gear ratio? And how heavy of a vehicle?

If it's in a B body with at least 3.55's or 3.73's and numerically higher, I'd do:

For intake, I'd go with an Edelbrock performer RPM. If that won't fit, knock up ebay for a holley street dominator to fit a 383/400.

If you're running stock rocker arms and do not want to buy adjustables, I would run a comp XE268H, if you want a solid cam, I would go with something of similar specs. If you've got the money to spend, roller cams are the ticket these days. Might want to scale back on the cam by one step if you've got granny gears like 3.23's or if you're in a very heavy vehicle.

Stock heads, or better yet stock heads ported with the MP porting templates will get you to your 400hp if you've got at least 9:1 calculated compression. If you want more flow and more power and shed some weight, a set of Eddy RPM heads or a set of 440 source stealths would do it.

Headers are a must.