"….with light loads as recoil could be "stout`"!…."
TH, you have the gift of understatement

70 grains of 2F Black & 450-480 grain bullet to start would be a good idea.

I have a Steven Grant .577 from 1873 that is a 3" but undoubtedly was one of the shorter cases when new. It weighs 8lb 4oz and really surprised me when I tried the period load for the .577 2 1/2' of 110 grains with a 560 grain bullet……only did that for 2 shots! Now use an ~85 grain and 480 grain bullet and it is much more "civilized". (can I say that word "civilized'? White privilege and all you know)

Cannot imagine a "period correct" .577 3" load of 160+ grains of black and a 560-590 grain bullet…….

My guess is that the conversion to the 3" case explains why the bores are in such good shape….the owners shot them little if at all.....did not want to loose their teeth and break their noses with their thumbs.

- Mike