BSA1
member
I ordered the 7 1/2" Blackhawk from Grab a gun in 45 Colt yesterday.
You should consider installing a Ruger Bisley-style hammer on your new gun. It lowers the reach of your thumb so you do not have to break your grip when recocking the gun. It is a drop-in part replacement.
As for shooting off of a horse I am surprised so many folks do not know about Cowboy Mounted Shooting. My wife and I have trained several horses and participated for several years. Never got bucked or had any problems with the horses. But, and a big but, is not all horses are suitable.
However you do need to train your horse to become use to gunfire when you shoot from the ground. Otherwise he might become frightened and bolt leaving you with a long walk packing your deer.
With a long barrel revolver you may find crossdraw holster more comfortable. I find it a crossdraw makes it easier to get on a horse.