Mingh,
The basic frame dimensions are the same. The rear fixing point is accessable once the grab rail is removed, correct. You will need to remove the lock assembly from the frame and add the front mounting tangs, quite simple actually.
Converting the 3C seat to hinges is definitely more challenging. As Grant has pointed out, the 3CL seat is a bit wider in order to accomodate the hinges and the lock underneath. Cut the 3C pan lengthwise, widen it and fit the hinges, you'll have almost everybody scratching their heads...
piet