If I was to get this rifle to shoot, I would build up correct factory shells for it:

.400 Diameter unpatched bullet 230 or 255 grains 1 in 12 lead/tin alloy NO antimony
Patch to .410 use either 11 or 16 lb paper (either seems to work)
110 grains OE 1 1/2 F if 3 1/4" case, 82 for 2 3/4"
Standard primer
3/16" lube cookie under bullet

This should be very close, if not right on the money.