A .458"-diameter right cylinder displaces 41.66 grains of water per inch of length/height.
If Bob loads his .45-70 LT 0.3" longer than his .45-70 Govt. load with a .458-caliber bullet,
he has added (0.3 X 41.66) = 12.50 grains of water in effective capacity.
H335 to fit in that extra space will weigh more than that since it is denser than water.

At that length of COL, Bob has reached the end of his throat in the .45-70 LT.
With the .45-70 LT+, using a .458 WM throat,
further effective case capacity increase as well as greater pressure let-off are possible.
With the excellent accuracy record of the SAAMI .458 WM, what is to lose ?

Since Buffalo Arms Co. sells .451"- to .452"-diameter, swaged-lead slicks of 530-gr. weight, I bet it is no secret that they work well in .458"- to .459"-grooved barrels.
BP or duplexed BP only, with paper patch, of course.

Ron aka "Rip" for Riflecrank Internationale Permanente
NRA Life Benefactor and Beneficiary
.458 Winchester Magnum, Magnanimous in Victory
THE WALKING DEAD does so remind me of Democrap voters. Donkeypox.