The 35 Whelen is a sledgehammer. More so on a rifle shorter than 22". The 45-70 will knock down any animal on the continent within it's range. The 35 Whelen was designed as a large game stopper at longer distances than the 35 Rem. Its more at home with 300+yd shots on Elk, Bear, Moose, or Caribou and such. Suitable for whitetails, however damages more meat than the 30-06.
Considering your intentions, I'd flat out go for a 1895G in 45-70 or 450 Marlin, a 444 Marlin, or a 375 Win.
If your hearts set on a compact barrel in 35 Whelen, by all means go for it. However, the noise and recoil will make a 45-70 sound and feel like a 308 Win. IMHO
In terms of which donor, use the Mauser. I would never molest a pre64 M70.
My aspirations are simple.
Send the kids to college, hunt, and own more guns
USMC 2111 Armorer, Ret.