From my recollection of fs98, the fuel flow rate for the 737-400 is extremely accurate, so in the absence of a fuel-calculating feature like the one in the fs2002 flight planner, all you would need is some utility that calculates fuel flow and you would be in good shape.
Secondly, during your flights, observing the fuel flow gauge at the bottom of the instrument panel is a way of predicting the fuel flow for various altitudes on future flights.
Have fun
