I guess it all depends on what you want...there is a great deal of extra cost involved swapping to a big block, regardless of it's displacement. But if you're going to swap, finding a 440 would likely be just as easy, and you suddenly have several more choices when it comes to pistons. As to whether or not it would be slower than your 318, what all's in your 318? If you want power approaching big block level, just build up a stout 360 or even go stroker...you could even put a 4" arm in your 318 and get 390 cubes...that kit adds about $1500 or so to the short block cost but you get a bunch of cubes right now.