I think I read somewhere recently about Martin Hagn building a big action so he is surely set up for it.
You would do worse than looking at the new Steve Earle Fraser action http://www.steveearleproducts.com/fraser.html
That would make a sweet 577 and a little less $$ than the Hagn but in the end, the cost difference will be nil and it really comes down to whether your friend prefers the side lever or the under lever.
If you can, please keep us informed on the build.