Everyone is giving you answers if you have a gun that's the exception to the rule.
All Modern 45 caliber revolvers (45acp or 45colt), will have a .451 barrel and will want a .452 boolit. The mold you purchase will likely be very close to .452 But a boolit rarely drops from the mold perfectly uniform, that's why you want to size it...a Lyman sizer die that's .452 will do that.
>>>The time to start measuring things, is when you have accuracy problems or Lead fouling problems...to see if your gun is the exception, rather than the rule.
Now for the 45-70. Marlin 45-70 gun's are notorious for being oversized, so a oversized mold is your best bet. If I where you, I'd look for a .460 mold. My Lee 457-340 mold, drops a bullet slightly smaller than .457
...Now a undersized bullet can work, but it needs to be cast with a fairly soft alloy and be pushed by a Load that will expand the boolit to fit the barrel.
Good Luck