Hi, I'm sort of the new guy on the block, I guess.
The ole T-53 loooks great, I can almost hear the igniters snapping! You are correct about the fuel cell extension, they first showed up on the "Charlie" model, which later became the "Mike" model. (the picture shows a "Bravo" model) However, the fuel cells were integral to the fuselage behind honeycomb panels and had a dimpled engine deck on top.
The transmission cowling was sliced off to fit flush at that deck. Right at about "item 14" as shown on your drawing/link above. There is a panel line (correctly placed) on the on the kit so a quick pass with your favorite scribing tool will show it off just right.
Can't wait to see how the rest of the ship builds up!
Regards,
Able Audacious Army Aviation Above All!