Old Marlins generally ran fairly large on their groove diameter, but had fairly deep rifling. The bore drill spec'd for a .32-40 was .313" in old Marlin factory records. Unfortunately they don't list groove diameter, so I can't tell you what the norm was.
Most of my old Marlins and Ballards seem to have larger than expected groove diameters, and my .32-40's haven't been an exception to that. When I've slugged bores they generally run around .321"-.322", so not sure why your bullet mold is so small?

I have 3 Ballards in 32-40 with 330 groove diameter.

         Don

Marlin 1893 here with .3225, worn enough at the throat to warrant a .325 bullet --  but 20" of it's 26" is closer to MM's example.

My mid-century 32 Specials are .317 - .318,

My 1894 32-20 is 3.12, but that one is a Taylor reline.  Can't remember what it was before I relined it.

Afterthought:  I wonder, would the bump up of soft lead and Black Powder account for your .319, or the intent to PP to other than groove size?

Well, just for tabulation sake I'll update and say I added a Marlin Ballard #4 Perfection in 32-40.    Groove is a shade over .324.   Not so very worn either.

I was just digging through some of my old loading tools, and found a boxed Ideal mold that was made in the pre WWI era when Marlin owned them. It's marked with Marlin's name on the box, and end of the box is marked ".319-165" and below that ".32-40"