1984 318 i Fuel tank / Drive Shaft Query

We inherited a 1984 318i that had sat unused for about 8 years without much gas in the tank, as one would expect the fuel tank gathered condensation, and (of course) the tank collected rust debris and other what may be. We replaced the fuel filter, cleaned everything (injectors, fuel lines, etc.), and the auto fired up and ran like a scalded cat for about 5 hours. The car started cutting out and finally had to be towed home. We have been told to thoroughly clean/dry the fuel tank, and to remove the fuel tank for this process?. This is proving to be much more difficult than just "unbolting" the tank as the drive shaft is attached to the middle of the gas tank (the tank is divided into an adjoining tank of 10 gallon and 5 gallon capacities). I am asking for opinion on whether the tanks could be flushed in place? Do we really have to get that tank out?, or could it be possible to clean the debris by vacuuming?? Please be nice, I am new to the BMW "world" and do not intend to ask "stupid questions" :(

1984 318 i Fuel tank / Drive Shaft Query