I had a 98 4.6 L truck. It was rated at 225 hp, and was probably less than 200 (~180ish) rwhp. You'll have a huge hurdle to overcome to get to 400 rwhp. Probably makes more sense to do a Lightning engine swap or get one of those newer 5.4s with pi heads and start from there.

 

I also own a 1983 chevy el camino with a 305ci SBC and no body wants to build them because of its big brother the 350ci SBC and ive noticed allot of people dont want to build a unique engine. Maybe its the easier road to get there but when you get blown away by a smaller engine that had more thought put into it, thats cool!