Rule of thumb
A general rule of thumb is 35-70 fps per inch, shorter or longer than "standard barrel length. However, this rule is only for average, as each different caliber and load combination varies. And, the individual rifles can vary that much or even more.
I have seen three different guns with the same barrel length, firing ammo out of the same lot give 100fps difference in velocity. Not the usual case, but not unheard of either. Some guns are "faster" than others, to a measurable degree.
two inches over standard (24 vs 22 in) should give you about 150fps more velocity, with the possible exception noted above. This would translate to about a couple of inches less drop at longer ranges. (I don't have any data for the .450 Marlin), but using .45-70 tables, this would be about right.
So, it all boils down to what you are willing to put up with, to get what you want. 24" isn't noticably longer than 22" in most situations. 26" is noticably longer. So, do you want the rifle to hit as hard, and shoot as flat as practical, or will you sacrifice a little bit of downrange performance for a handier package? Long range magnums benefit more from longer barrels. Woods rifles get their utility from shorter ones. Your call.