The shank only matters for the tool that you are using. 1/4" shank will be fine for most routers. You should be worried about the width of the cut. This should be small for t-molding...on t-molding.coms site it is called "cutter height" 1/16 of an inch. You don't want to go much higher than that so the only one that you can us on MLCS's site is the 5/64"...it is probably the same bit as t-molding.coms bit. If you are cutting into MDF you will probably have to use some glue around tight edges accompanied with V-shaped cuts in the insertion part of the molding. Use tape to hold it in place until the glue is dry. This will give you the finished look you desire.
You are correct, it is 5341.
Hope this helps.
-G