10.5" to 11" Clutch???

The orginal build on my son's 68 Dart was 8.75 rearend that was going to be setup mainly for the street. We chose a 10.5" clutch from South Bend Clutch and also a flywheel from 440Source. Now that the car is going to be backhalfed and hp will eventually be more than the 400...it was suggested by South Bend that we use the 11" instead. Now that we have all the parts for the 10.5...what will we need to change it to the 11" flavor? We actually have both clutches sitting on the shelf in the shop..so one will need to go back and the disk of the other will be sent back to have the other side ceramic coated...
Ideas?

Rick