Isopeura,
IMHO,9.5LBS would be a good working point for your .400.
I have just spent the weekend playing with my 425WR on our farm. The rifle which weighs 9.5lbs is quite comfortable and you can almost get away with just holding it in one hand by your side all day, yet I find it holds the recoil down well.
A 63cm or 59cm long barrel?
Congrats on your project.
I hope you get the opportunity to hunt Ele with it.