YOu will have to measure the diameter of each of the wads or cards you buy. Usually, the diameter is a little more than the bore so you have a tight fit. I just measured fiber wads for my .50 cal. rifle and they measured .510", or ten thousandths over bore diameter. These are also from Circle Fly.
As for killing birds, penetration is a function of the weight of the pellet, and not the velocity. Velocity is shed so fast, that if you want more penetration at any given range, you have to use a larger size shot. I use #5 shot, rather than #6, because It penetrates much better at all ranges out to 40 yds.
Killing birds is not the result of putting one pellet in a vital area. Instead, we knock them down and kill them with the shock of being hit with seveal pellets. Birds with heavy thick feathers are difficult to kill with any body hit, so we shoot for the neck and head, hoping to penetrate the neck or head, or at least knock them out. This applies to turkey, canada geese, and some other species of geese. For upland birds, like pheasant, quail, dove, grouse, chukkar, etc. and squirrels your load of #6 shot will do fine, if you stay within 25 yds. 75 feet is a long shot to be taking at most of these birds. I have killed birds at longer distance, even with an open choked gun, but I had the benefit of plastic shotcups, that tend to help close patterns. With a choked shotgun, I have killed birds beyond 40 yds, and see other shooters make even longer ranged kills.
If those pellets went through 1/4" of solid wood, that is a lot of energy to dump on a small bird. I would not worry about whether that load will kill. You might try shooting that load at a vegetable can--- a real steel or tin can, and not an aluminum pop or beer can--- placed at the distance you intend to use as your maximum range. If your pellets can penetrate both sides of the can at that distance, you have an adequate charge.
If you do have access to a chronograph, and want to test the velocity of various wad combinations, simply make a paper tube the size of your bore diameter, or slightly less, crimp and tape the bottom end, pour your shot load in that tube, and turn the top edges over and tape them. You will have made a " shot slug ", but it will be the same weight as your normal shot load, and you can send it safely over your chronograph screens at various ranges to determine your velocities, using those different components. It doesn't have to be pretty, for this use. All it has to do is stay together so you don't shoot up, or kill your chronograph! Do use an adequate backstop for these tests, Please.