I built two into 45-70 a No1 MKIII and a No4 Longbranch I turn the rims to .303 diameter , as I remember to .550 dia.Might have to check that as I don't have a cartridge in front of me. That way you do not open the receiver and it keeps the bolt head secured.

Article I once read about converting to .45/90, the fellow kept bullets to 350gr. Hornady and the lighter 300gr. Nosler Partition. He put slips of delrin (or nylon)in the magazine to convert it to single stack. This worked perfectly, for him.
That was back in the late 70's, maybe early 80's but I cannot remember which magazine- could have been Rifle/Handloader, Shooting Times or Guns & Ammo).

Daryl..I have a copy of that article somewhere if you are interested.

This is the longbranch