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I've recently replaced two 15.8 gallon fuel tanks with 19.8 gallon tanks on 740/940s with live axle. The 740s and 940s made prior to 1993 had the 15.8 gallon fuel tank. 1993 and later 940s have the 19.8 gallon tank. To install the larger tank in your earlier car you'll need the tank, the Y shaped fuel hose which connects from the chassis to the late fuel tank, the front and rear fuel tank support brackets and two straps, the front half of the hard fuel line running to the main fuel pump, about five feet of 1/2" fuel hose and two worm drive clamps for it.
I recommend running the fuel tank pretty low before you start this project. You'll want to remove the fuel pump and level sending unit after you've removed the fuel tank. Installing the unit with the tank out is a lot easier. If you've never replaced the in-tank fuel pump, this would be a really good time to do it.
Below are the two fuel tanks. The Y shaped hose you'll need is sitting on top of the larger fuel tank.
Once you removed your old 15.8 gallon tank you'll need to remove the large fuel line and it's two supports. The supports are circled in yellow. The fuel line is in the way of the larger tank.
The small fuel line and the vapor line may need to push over a little to fit the new tank. You'll see what I mean when you try to fit the large tank for the first time.
The picture below shows the large fuel line installed through the driveshaft opening. It is the green tube. You could use rubber fuel line for this, but I like to use the hard line for getting past the driveshaft and up past the exhaust system.
The picture below shows the rubber hose running on the inside of the rear subframe that keeps the axle housing stabilized.
The picture below shows how I secured the large fuel line behind the main fuel pump.
The large tank can be installed the the car jacked up and having the chassis supported on jack stands. I recommend loosely installing the front and rear fuel tank support brackets and then rotating the tank up and over the driveshaft. One person can do it, but it sure goes easier with two. Tighten the bracket bolts up after the tank is in place.
I've installed two of the larger fuel tanks in earlier cars. One in a 1992 944T, and the other in a 1987 745T. The sedans and wagons use the same fuel tank. IRS cars use a different fuel tank and you cannot use it in a non IRS car. The pocket in the fuel tank, which the fuel pump and float fit into, is a little different between early and late cars but the fuel pump and float assembly from the early tank fit and function fine in the late tank.
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john
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Replacing a 15.8 gallon fuel tank with a 19.8 gallon tank [700]