Others will comment on needles and tuning and the like... But I would like to say that the flange thickness (surface touching the head) is going to be different on the early intake manifold and later exhaust. Some folks use half washer and the like. If you have access to a machine shop, bolt both manifolds to the table of a mill (in the same interwoven style like they would be on the engine) and use an end mill to cut them to the same offset distance. This really makes it easier to install rather than odd ground washer and the like... I did this on my '64 1800 that has an early intake manfold (standard alumn for SU's) and a later double down exhaust...