Have you ever built an engine before? If not then you might consider starting smaller. Modding a 350 block for a 400 crank can be a lot of tedious work especially if you've not done a rebuild before. That's not even taking into account the cost, it can be a lot more expensive than you might think. But if you've got the time, the money and most importantly the drive, I say go for it. Nothing feels better than accomplishing that kind of build.