How big a bullet will a fired case accept by hand seating? That is likely going to be your best size for the finished product. Most likely you aren't going to have to resize the cases or use any dies on them except possibly a taper crimp die - see below.
No need or want to crimp for a single shot rifle. Maybe a slight taper crimp if the bullets want to fall out.
For powder, figure out what is safe with a 100% load to the base of the bullet which is seated to engrave the rifling and use that. (Set it up so the cartridge is seated the last 1/16" by closing the action)If accuracy isn't what you'd like drop the powder a little and add some filler - cream 'o' wheat or BPI shot buffer. No dacron, you want something granular that will seal the bore.
Just running Quickload with this combo it looks like 100% 2400 is safe, as is H110, 296, Lil' Gun, Ramshot Enforcer, AA4100, and many others that should work well in your application. You might see 2100 fps, but likely not much more at full whack. Still a rather potent combination, especially since those bullets are nearly pure lead and will expand violently at that speed.
