Nitro - weigh the slugs with their wads, then start with shot data of the same weight (giving for example, 8,000LUP) - I'd use SR4756 or 7625. 800x should also be OK due to being a 'slower' powder. I'd not use 700X as it is a trap powder meant to give low (trap shooting) velocities at peak pressure for the bore). The starting data (Lyman) should give a very safe starting point, then you can increase as pressure signs and velocities show. This is how I devloped my own data when starting with round balls and no printed data.

I use this same procedure for all unknown projectiles in the bore guns.