Building a custom chopper involves understanding transmission mounting, especially when using a hardtail frame. It's common to think engines have built-in transmissions, but many choppers require separate mounting. For accurate alignment, it's recommended to bolt the mounting plate to the transmission case and ensure the primary drive is aligned before welding. Consulting custom motorcycle shops or Harley-Davidson dealerships can provide valuable insights. Proper measurements and leveling are crucial to avoid complications during assembly.