I do not have a Hodgdon Manual that shows black powder loads, but 1670 fps with 90 grains of Goex 2F is faster than my most of my tests show. The Lyman Black Powder Handbook and Loading Manual, 2nd edition, shows a .54 32" barrel, 1-60" twist,.535 RB, 90 grains of Goex 2F producing 1480 fps. (And 90 grains of Goex 3F getting 1475 fps!) I tested this to see if it was so. But first, here are some 90 grains of Goex 2F and round ball velocities from some rifles I built:
a 42" barreled Christian Hawken caplock, 1606 fps.
A 42" barreled Henry Albright flintlock, 1636 fps.
A 32" custom LH Bridger Hawken, .535 RB, 1521 fps, .530 RB 1479, 1546, and .520 RB, 1500 fps.
My 34" Green River Leman, .530, 1641, 1694 and 1738 (heavy patch).
My 32" custom Bridger style caplock Hawken, old Moosic, PA (1978) powder, 1638 fps, new mfgr Minden, LA powder, 1515 fps.
You can see photos of groups shot with 80 to 120 grains of Goex 2F and 3F in this Hawken, with velocities, in the General Muzzleloading forum, my post "Comparison of Goex 2F and 3F Velocities in a .54 Hawken", 12/21/08. Post number is 657638. That barrel liked 110 grains of Goex 2F. I'd go for 110 to 120 grains of Goex 2F.