Mr. Phil,
In my 24" Marlin Cowboy 5.8 Trail Boss / 250 cast bullet = 771 f.p.s. If the powder was placed to the back of the case prior to chambering - 816 f.p.s.
As Ranch 13 indicated, Unique, Blue Dot, 2400 would be much better choices for hunting rounds in your '73.
The Alliant recommended load of 8.0 / Unique (safe in a '73) generated close to 1,100 f.p.s. in the 24" Marlin (250 gr cast)
The Alliant recommended load of 11.5 / Blue Dot (safe in a '73) would likely be close to 1,200 f.p.s. with a 250 gr bullet
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