I am in the process of rebuilding my rear brakes on my 340.
I ordered a standard set of brake rubbers from SNG Barratt only to find the off the shelf item was the wrong size.
The standard piston size for the 340 is 1.5 inches. My piston are 1 5/8. This was the standard item fitted to the mk 10 and XK 150.
My advice is to check the size of pistons before ordering the replacement parts and hope you get a sensible sales person from your supplier.
Regards Terry