FH,
Take care with how you read the limitations. You are trying to mix cargo limits with a fuel tank structural limit, which is different. The tank is individually certified to its full capacity as mounted structurally on the forward wall of the baggage compartment, as well as on the floor, and its capability is not limited to the weight bearing limits of the baggage compartment, at all.
The tank has 106 US gallons usable fuel (from memory) and about 110 gallons total volume, and the tank weighs about 100 lbs, so it will weigh as much as 840 lbs total, fully fueled.
As I recall, the tank is covered in a suppliment to the RFM, perhaps a call to the local Sikorsky rep (or the factory) would unearth a copy.