Stewart platform (SP) is a promising choice for large component alignment, and interactive force measurements are a novel and significant approach for high-precision assemblies. The designed position and orientation (P&O) adjusting platform, based on an SP for force/torque-driven (F/T-driven) alignment, can dynamically measure interactive forces. This paper presents an analytical algorithm of measuring six-dimensional F/T based on the screw theory for accurate determination of external forces during alignment. Dynamic gravity deviations were taken into consideration and a compensation model was developed. The P&O number was optimized as well. Given the specific appearance of repeated six-dimensional F/T measurements, an approximate cone shape was used for spatial precision analysis. The magnitudes and directions of measured F/Ts can be evaluated by a set of standards, in terms of accuracy and repeatability. Experiments were also performed using a known applied load, and the proposed analytical algorithm was able to accurately predict the F/T. A comparison between precision analysis experiments with or without assembly fixtures was performed. Experimental results show that the measurement accuracy varies under different P&O sets and higher loads lead to poorer accuracy of dynamic gravity compensation. In addition, the preferable operation range has been discussed for high-precision assemblies with smaller deviations.