The 300/400 srs had the centre fuel tank that gave them a better range than the 200 that only had wing tanks. The 500srs had the same fuel as the 300/400 but had more airframe to carry around thus less range.
IIRC the wing tanks held 1100 imp gals and the centre tank held 800 imp gals.
P.S. The 300 and 400 series aircraft were the same other than the 400 was certified at a lower max weight for US certification with a two crew cockpit.