I am running a 3.55" round in a 3.7" box with zero feeding issues using Hybrids.
If it does hang up, you could add a block to the rear of the mag box.
I have a Kimber in 300WM that originally had a 3.4" box, I changed it out for a Talkeetna box/follower/ejector so I could seat out to 3.6". Gave me fits with bullets being driven into the top of the action, the cases were jumping out early...so I put it back to how it was. Now I have $300 in spares for my Talkeetna that never gets used. Hahaha.
Cheers.