Is your 1858 Remington a repro or an original gun?
There can be HUGE differences in dimensions, the new ones generally being a lot more consistent; they really didn't have that much of a handle on tolerances in the States yet. The FIRST industrial product to be made with PRECISION and with 100% interchangeable parts was the 1858 Enfield rifle. EVEN Colt handfitted parts for best function although the parts were machine-made.
If your gun is an original, don't even think about hard lead. Save that for your modern rifles; it works fine at higher velocities. I use wheelweight metal in Tokarev, Steyr and Luger pistols, also in my SMLEs, but my Martini and Sniders get only the softest, buttery metal I can find.
BTW you CAN de-alloy your wheelweight metal to a great extent by melting it and NOT fluxing it. Let it remain molten at heat and skim the Antimony off the top, then pour off the bottom, melt another batch. Stuff you pour off the bottom will be much softer.
For an OLD gun, you need stuff that you can groove with your thumbnail.
Hope this helps.