AFAIK= As Far As I Know
The dimensions of the GT and G40 cranks are the same, although having seen turnings off each crank after them being lightened, I'd suggest they're slightly different, without taking the turning to a metrology lab I don't know how- be it a slightly different alloy, they're both still cast then machine finished though.
The webbing on GT blocks is different to that of a G40, this is only external (from memory) but must add strength, I don't think anyone running a 3F block has had problems though- although you will have to take into account the oil return feed for the charger, and the knock sensor boss at the rear of the block.