I use 40 grains of IMR 4064 in my Ruger No. 1 .45/70 to duplicate my black powder load of 65 grains of Swiss FFG and the Lyman 457122 HP which casts at 342 grains from WW metal. The MV of both loads is 1350 FPS, and of course, the IMR 4064 load is much cleaner! However, both are accurate. I shoot the bullets ungreased, but with a greased .45-cal Wonder Wad on top of the powder. I have even shot some of these made from pure lead with the Wonder Wad with no trace of leading. The pure lead slugs shot well, and would no doubt be a good short-range deer load!

No doubt your MV would be at least 100 FPS higher using that long barrel.
I think your proposedload is a good one, but if that bullet has a step for a gascheck, I would use one just to j=keep the powder gases off of the bullet base. When I load hot with a flatbase bullet that has no provision for a gas check, I put one in upside down on top of the powder before seating the bullet.

Let us know how it works out.