A 400 grain bullet in .45 caliber will have a higher SD than one in .50 cal., .220 vs .272, which would increase the possibility of the .458 projectile penetrating deeper. A .458 of comparable SD to the .50 would have to weigh about 325 grains.
An LBT bullet in .50 cal would certainly have the meplat to hit hard!
I've used Beartooth Piledriver Jrs at speeds up to and slightly over 1800 fps in my Guide gun, the penetration is tremendous, and the Piledriver (525 grains advertised, mine weigh in at 540 grains) at 1550 fps dig really deep. Neither load is over 36 Kpsi, so there's more on the table.
Recoil can be a real snot-knocker...
Personally, I'd just stay with the .45-70 in the Marlin platform. I'd have to have something that the perceived recoil was not so vicious with to go to any higher performance level.
