It should be between the rear axle and the fuel tank. It's fairly easy to do. I wouldn't attempt it without a shop manual... that would have told you where it is
MB shop manuals are preferred but a Haynes manual covering the 450SL/C models should be a reasonable guide for your car.
A decent manual will provide the necessary precautions for working on the pressurized fuel system, and it's pressurized even with the engine off even with a bad accumulator.
Don't forget to replace the crush washers on the banjo fittings.