Does it have screw in chokes? If so , I assume that they are invector and not invector+. Let us know.
There are several 2 3/4 " loads available but in the 20 I suggest #6 or #7 shot. In lead you will find #4, #5, #6, or #7.5. in some loads to get the best pattern you may find yourself very limited in distance. The smaller shot tend toward better patterns, especially when using 20 gauge 2 3/4" shells.
I have some HTL in H-13 #6 shot and Federal HW#7 shot in 2 3/4" but you may not want to use HTL in your gun.