[Bruce Lancaster]

On manifolds: The aluminum short/iron long rule is generally true but not 100% so on Ford manifolds. On aftermarket, about everything is aluminum and both of the '35-48 heights can be found. SIMPLE way to sort, wish i had exact numbers written down, but since there are only two common choices this is plenty: Measure from the bottom of manifold casting at the rear edge to the machined flat where the fuel pump stand lives. If that is in the general neighborhood of 1" it needs the short, if in the general neighborhood of 2" it needs the long. Once you notice this you don't need to measure...you should be able to tell just by eyeball. '49-53 manifolds are all one height. if you need to use one on an early motor they are correct for early long pushrod. If you are disguising a late motor as an early so it will look right in a hotrod or race car, you can convert the block for early pushrods by installing the early bushing directly into the late bushing.
Up front on earlies the stud to mount generator/fan to manifold comes in several heights, enough different to require different fan belts. Aftermarket manifolds also vary there.