In mass production, planning for customized products is difficult to achieve against the effort toward standardization of parts and processes. In a complex product such as a vehicle, varying an entire subassembly multiple times per shift increases the margin of error for defects and slows the production rate. To understand the effect of assembly instructions and part organization on assembly time as a result of process variation, this study examines building different subassemblies of two different models to simulate a customized production line. Assembly time is measured while varying the work instructions, part organization, and subassembly type, and relationships among these variables drawn. Picture instructions result in shorter assembly times for all subassemblies, while line configurations for shape and color part organization have the shortest assembly times. However, only picture instructions for two out of four subassemblies show a statistically significant effect on lowering assembly time characterized by a p-value below 0.1. Based on a linear model, total product assembly time is predicted within a 20% margin of error, based on which subassemblies are built and instructions used. Based on these preliminary tests, a better experimental plan is needed to isolate test variables for model creation. In addition, more experimental data is required to improve the statistical significance of the results. A heavier emphasis should be placed on the cognitive factors of the assembler in assembly production planning.