You really need to use a wider diameter cast bullet. Marlins have a reputation for having large groove diameter barrels. One Marlin rifle in 45LC that I know of didn't shoot well with cast bullets until the owner used 458 diameter cast bullets.
NOE list 434 diameter moulds or you could design your own mould using the Mountain Molds program. Specialist suppliers like Buffalo Arms and Tracks of the Wolf have a range of oddball sizing dies. I have had a few sizer dies made by "Buckshot" on the Castboolits site. He will make any non standard size - his work is excellent and his prices are very reasonable.
