Hi there, have you tried trimming/shimming the foil slightly to reduce the rear foot pressure you are feeling? Maybe check on the Armstrong forums/Facebook pages, I know they give some advice there on trimming?
Adding inserts is no problem, we do not sell them ourselves, but most custom shops can just add any or sell you some - most important is obviously to use the same screw type/drill the inset open with the same drill bit size so the screws fit properly.
I use the 4'11" mainly, sometimes the 4'9" if it's windy, 183cm, coming from 40+yrs of windsurfing, it does take some getting used to in winging in general with the wider stance I must say.