There would appear on the face of it to be little point in using such a volumous case to drive 400 gn heads @ 2000 fps.
There are loads that will get close to that in a 45-70 case around the 35,000 mark and will take the uncertanty of fillers out of the equation.
If it is a fun and hog gun that will be plenty enough.

Personaly I would regulate it for an availible common 45-70 factory load and then build a handload that duplicated the balistics and regulates the same. Then shoot the hell out of the gun until you are realy good with it. The superiour bullet placment will more than compensate for a slight theoretical loss of power.

Regards