Terrific question @perlake! In determining that culture integration will be difficult, you should have identified specific factors – values, decision processes, command/control structure, cultural flexibility factors, L&D tolerances, tech-savvy, etc. – to which you can point and show where the challenges lie. It is recommended that you just be very direct at pointing out the respective challenges first and foremost. If the benefits of the merger are significant enough, as you shared they are, then the next step would be to identify solutions to each of these challenges. Always come to the table with a set of solutions. Those may not work or they may not be the final set of solutions, but they will start the discussion at the very least. Be sure to engage in open dialog with the deal team and the represented HR teams from the merging companies.
Communication is the most critical tool. Crafting appropriate messaging, communicating the message as transparently as possible, with empathy towards the respective audiences, and adapting the messaging to match the respective audiences will make or break the integration. It is strongly recommended that strategic planning be part of pre-integration, that the outcome of the strategic vision and mission align with the merger thesis, and that appropriate unified values be determined that pull from all merged entities to reflect true unification of the businesses. Each audience will need to understand WIIFM (What’s In It For Me). The unified organization needs to come up with methods for integrating systems, processes, procedures, policies, total rewards (and corresponding philosophies around compensation & benefits), organizational design, command/control structures, decision processes, and develop appropriate retention strategies for key talent. The treatment of people exiting (due to duplication of resources, lack of cultural alignment to the future state organizational culture, or other business cases, whether voluntary or involuntary) should be managed with the highest integrity, providing appropriate severance packages based on level, tenure, behavior and performance factors.
Its not a succinct response, but neither is your question free of nuance. Hopefully that provides some guidance, context, and vision for how to proceed.
