I have two recipes that I use for a couple of old Parkers:
12g. Federal paper 2 3/4, 17g Red Dot, CB 1100 wad, 1oz lead. ~5000 psi
16g Cheddite 2 3/4 plastic, 19g Solo 1250, SG16 wad, 1 oz lead. ~7100 psi
Could bismuth be substituted (volume due to weighing a little less than lead) directly?
As long as you keep the shot charge at or less than 1 ounce you will be OK. If your shot charge is less than 1 ounce due to volume difference your chamber pressure will be slightly less and your MV will be slightly higher.
By the way where are you finding Solo 1250. I haven't seen any available for a couple of years?