IMHO bushed firing-pins are less necessary these days than they were a century ago. Modern primers rarely pierce, and a new chamber and modern brass should never let gas escape. However, if it ever did happen, you will almost certainly split the stock at the head, and in the worst case, could lose an eye. Your choice! The Scott-style gas-escape grooves in the standing-breech might not help as much as the original designer intended, but they would certainly be better than nothing. In any case, it will be necessary to examine the firing-pins very closely and make sure they are properly rounded, and don't protrude any more than necessary.

The best nitro-for-black loads are straight out of Graeme Wright's book "Shooting the British Double Rifle". Sounds like your library lacks a copy, which you will need to rectify! Mandatory reference for loading doubles chambered for Brit cartridges, both BP and nitro.

70-grains Varget beneath a 350gr jacketed projectile will regulate in 99% of doubles chambered for the .450 x 3 1/4 Express. A polyurathane-foam or saddle-felt wad (or two!) will be needed to take up the air-space. Velocity should be around 1950 to 1970 fps, and chamber-pressure will only be around 85% of that generated by the original BP loading.