You really do not "need" a 4 bolt block for a 400hp engine. If and when you decide to step up the power you could always just put splayed caps on and it will be stronger than a factory 4 bolt block anyway. I personally look for 2 bolt main blocks. Splayed caps are not all that expensive.
Since you are building the engine from the bare block why not just build the 383 to beging with? It really isn't going to cost you any more. It will be cheaper in the long run, if you build the 350 and a few months down the road build the 383, you really don't save any money at all.