If it's a 3 3/4 dram rifle around 110 grains of Olde Eynsford 1 1/2F will get you close.
Swiss doesn't have enough oomph.
Use a .450 dia paper wrapped bullet 1 in 15 to 20 would be good and about 300 to 310 grains.
Don't worry about measuring the bore - it'll fill it. Grease cookie 3/16".

The rifle was probably regulated with 270's so the above load may shoot a little over the bead, but it'll be very close at 100 yards.

The load will tell you what to do.

Forget NFB's unless you want to try ringing a chamber.