You have the direct opposite problem of many Pietta '58 owners who want to use conical bullets. Their solution has been to go to VTI gun parts and order an Uberti plunger and swap it out for their Pietta one. They all said they fit fine and they just remove the one screw, swap the parts and put the screw back in. I'll bet if you visit the '58 Remington.com website, introduce yourself as a new shooter, explain your situation, and offer to swap plungers that all it will cost you is postage each way. I hope this helps. GF