Buffalo Bore loads a 45 Colt that is safe in all Colt Clone pistols. If it's safe in them, it should be ok in your '73.
As far as getting a large increase in velocity when shooting pistol loads in a rifle, you get them but not as large as you think. There is still only 14-20 grains of powder pushing that bullet and the pressure curve has peaked long before the bullet leaves the barrel.
