The three original tanks in our boat were replaced by the previous owner 18 years go with five smaller tanks. This did not require the removal of the engines although I suspect the transmissions had to be removed. The original tanks were cut up in place and the pieces removed.
In place of the original three 150 gallon iron tanks there are two 85 gallon saddle tanks on each side of the boat and a 60 gallon day tank in the bilge just aft of the engines. All five tanks feed from their lowest points via gravity. There are no pickup tubes and no pumps in the system other than the lift pump on each engine.
In practice, both engines and the generator feed from and return fuel to the day tank. Fuel from the saddle tanks is transferred to the day tank via gravity from the saddle tanks with manual valves. Our fuel management system has us transferring from from one opposing pair of saddle tanks into the day tank as needed until the saddle tanks are empty. We then start transferring fuel from the other oppposing pair of saddle tanks until they are about a quarter full. We then fill the two empty saddle tanks, finish emptying the two tanks we've been using and then leave them empty until the first pair are down to a quarter full, and so on.
The system is such that each engine can draw fuel from the day tank (normal setup) or from either of the two saddle tanks on the engine's side of the boat. The fuel return from each engine can be directed the same way.
The generator always uses the day tank .
Finally, fuel can be transferred between any of the saddle tanks using a separate transfer pump. This pump can also pump fuel off the boat from any tank. However we have never used this either of these capabilities in the 17 years we've owned the boat.
In summary it's a great system that is simplicity itrself but with the capability of moving fuel around if required. And because each tank feeds from its lowest point, when a tank is empty, it is totally empty.
The only negative about the whole system is the tanks are made of stainless steel. They were very well made by a speciality shop in the SFO bay area, but stainless is not the best choice for a fuel tank.