This study focuses on model-based system level optimization of a fuel cell electric Class 8 long-haul truck application using GT-SUITE and Matlab/Simulink. First, a 1-D propulsion system model for a class 8 heavy-duty vehicle with a conventional diesel powertrain was developed and validated against test data. The model was then modified to develop a model for the fuel cell electric propulsion system by utilizing component level test data. To ensure optimal control for each hardware combination, an ECMS-based adaptive control strategy was developed and implemented for energy management. Powertrain components such as fuel cell, HV Battery, E-Axle motor and gearbox, etc., were all parameterized to perform optimizations to identify the component specification that meet performance, efficiency, weight and packaging requirements.