Off of MD Smith's reloading page, this was the closest .45-70, with a 300gr JHP: Varget 62.0 gr. 2,090fps (Compressed load)
I believe some of the Buffalo Bore loads make that look tame.
Edit: They have some SMOKIN' loads! BUFFALO-BARNES LEAD FREE 45-70 MAG +P 300 gr. TSX-FN (2350 fps)
300 gr. Jacketed Hollow Point (2,350 fps/M.E. 3,678 ft. lbs.)