45/70 in a short, light, bolt action = PAIN, bad idea, and with the existing Marlin guide gun type 45/70s an unneeded solution to the "handy rifle with a big punch" problem.
the sumit carbine is a marketing ploy not a truly useful gun. it's up there with the freedom arms ans similar 45-70 revolvers. and my even be in the same class as that insane single action six-shooter that some joker made up in 30-06!!
maybe the thing was made for those who liked the Remington-keene concept of a 45-70 bolt gun i don't know but the mere thought of that round in a rifle that light OR a stock that narrow much less both at the same time is painful!!
if you want a nice short handy 45-70 get one of the carbine length Marlin guns. the gibbs while intriguing is not exactly a gun most folks will find comfortabel to shoot.