Every 1983-1984.5 760 TD that I have seen has had the extra range fuel tank. I had always thought they all just came that way, but Tyler you're saying your '84 did not have one until you added it yourself?
Agree on the transmission, the ZF autoboxes is a great setup in the 700 series TD cars, very refined and an ideal match to the D24T power curve, and remarkably efficient as well, almost no penalty in mileage compared to the M46.
I'd bet that when you get the car on the road with a good motor and healthy ZF behind it, you're not gonna want to change anything about it.