I would not worry about factory ammo OAL for a modern custom gun. Your head throat / lead & magazine length are the critical factors that should determine OAL. In other words, the bullet should be seated about 0.030 inch from rifling & still fit the magazine. If that makes it too long, you should still be able to get it in the magazine easily for smooth loading.

My Simson rifle was made before 1912 according to the proof marks.

My loads' OALs are
* Lapua Mega 285 gr RN 3.229 inch
* Norma 286 gr RN 3.244 inch
* Woodleigh 320 gr RN 3.180 inch - this is a fatter & longer bullet that needs to be seated deeper in the case. The ogive is further forward & hence the OAL has to be shorter in order to clear the rifling by 0.030 inch.

I have made up dummy rounds with bullets in empty cases for seating die set up. I used 30'06 cases & they feed fine!! You could just load up dummies in the Norma cases & check the lead / seating depth.

Hope this helps. I would be interested in what you decide. Good luck.