The .425 Express is based on the .300 Win Mag. Correct?
It might be worthwhile calling and discussing with Martini. I've heard he also does more routine gunsmithing.
Personally though, I would be calling Bill Leeper in Elko, BC. Bill is very good with Mausers and their feeding. He built a .375 Ruger for me on a FN action that is the finest feeding Mauser that I've ever owned. The action was originally in .270 Win so there was a fair amount of work on the rails, feed ramp, follower, etc.
As a bonus, his prices and timelines are usually very reasonable.