.45 acp is probably not an ideal caliber for a combo
You may squeeze in two extra rounds but you may not see an appreciable increase in velocity or range, if you use the same load as the pistol load.
To get higher velocity from a rifle barrel, you need to use higher pressure .45 acp rounds, using slower powder, like those designed and made for the Thompson sub-machine gun. But then you'd lose the benefit of ammo commonality with your pistol.
Your accuracy may benefit from the increased sight radius, but if you are a decent shot and accurize your .45 pistol (1911 type), then you'll probably hit the same groups (6"-8") with both guns, up to 50 yds.
Need more firepower from your pistol? No problemo, get a hi-cap double stack 1911-type, or 10-rd magazines for a single stack 1911.
IMO, the .357 magnum is the best candidate for building a revolver/lever combo, followed by .44 magnum. Real benefits can be had with such combos.