If you are using the Speer shot capsules, fit the over all length to your cylinder. They wouldn't tell you to load 5.5gr of Unique if it wouldn't fit properly. You should be fine.
I've taken grouse at some pretty good shotgun distances with 357mag shotshells, though closer is better. Made me a believer many years ago. My brother in law used to say it wasn't fair that he carried a 12ga pump and all I had to carry was a little revolver, and he never got to shoot because the revolver came up too quick.
Later, I read Elmer Keith's method of building shotshells for those who don't have factory shotshells available. My Dad had the same reaction as a lot of folks. "Might as well throw a shovel full of sand at them!" was his sommentary, until "that day". We were deer hunting and about noon I spied a grouse, which became a covey of grouse by the time I saw all of them. I had my 41Mag Ruger Blackhawk loaded up with #4's and a charge of Unique. By the time the gun was empty, I had my daily limit.
Made a believer of him, also.
Load and enjoy.
Regards,
Sweetwater