I carry and shoot a M638 J frame all the time, and do so fairly well. How much more difficult will a 3.5" Colt replica be to shoot?
You didn't mention at what distance you “do fairly well” with the little S&W 638, but I presume you will do as well or better with a 3 1/2" single action.
So far as accurate shooting is concerned you lack weight and forward balance, and will get some vertical stringing of shots as a .45 Colt round rolls and rotates the muzzle upward in your hand. Also after the chambers get fouled it's a pain to eject the fired cases.
Here in Arizona a 4 3/4" revolver is considered to be good to at least 100 yards. I have made some admittedly lucky shots at twice that distance. It's a matter of knowing your trajectory. The .45 shoot like a mortar, but once launched nothing disturbs that 250-grain bullet.
Using a Colt 2nd. Generation .45 / 71/2” I’ve knocked down metallic rams (about the size of a average German Shepard) at 200 meters, and many others have done the same. My score at that distance with any S&W J-frame snubby isn’t particularly impressive…
The Old Fuff realizes he’s over the hill, and few pay any attention to either his advice or rambling. That’s why I mentioned the availability of an excellent 3 ½” Sheriff Model at Dixie Gun Works.
Sheriff Model single actions are sort of limited in they’re potential, although somewhat fun to play with. I still stand with my suggestion that you start with a longer barrel length first, and get the shortest one last.