UtopiaTexasG19: a 255gr slug doing 860fps packs 419ft/lbs of energy. Google:
ballistic energy calculator
...and you'll find several. Some will also tell you the "Taylor Knockout Factor" if you also punch in the width and shape, or tell you the drop over distance if you know the aerodynamics of the slug ("ballistic coefficient"), but all should tell you basic kinetic energy in foot/pounds.
Now, none of this tells you peak pressure. That very old-school Winchester load is *probably* peaking at 14k or less though, it's right in the ballpark of a round that does that.
BUT, you need to know something important here - esp. if you start handloading:
You can get (or brew up) a "stupid load" that doesn't put out a lot of bullet energy BUT does a short, brief "spike" in pressure that can blow the hell out of the gun. Follow? What hurts the gun is the peak pressure, not the duration (within reason of course).
So, a really intelligent load that uses exactly the right powder charge might peak at "only" 14k, yet hold that pressure for significantly longer than a less-optimized load. In extreme cases a 14k load might well put out more energy than something dumb that's peaking briefly at 25k or more.
One way to do a "low but broad pressure curve" is to use a heavy hardcast bullet and slower burning powder. The heavier slug takes longer to start it's motion (we're talking about tiny fractions of a second, but it matters). That in turn lets a slow-burn powder "catch fire" properly, and then it shoves the sucker out very nicely.
The downside is, you have to use more (sometimes a lot more) slow-burn powder. Most powders weigh about the same, so a fast-burn-powder recipe will be literally more fuel-efficient - lower cost recipes, less powder. And heavier bullets cost more.
Anyways. I strongly suspect those Winchester Super-X loads will be OK...but I don't know for sure. Personally, if it was my relic, I'd buy good quality cowboy-match loads from a good online source like Black Hills, who make some of the most consistent ammo on earth.
Expensive? Yeah. But that could well be a $5k gun. And look at what their speed rating is on 250gr 45LC cowboy stuff 750fps:
http://www.black-hills.com/cowboy_action_calibers.php
They also have 45Schofield loads which are basically slightly shorter and will work in that gun, and are even milder...