Someone told me that a fast 78 could be converted to a 90mm if you bought the 90mm upper? Is the true? Thanks

 

as far as i know and have read it is true thats why the upper shell is over 400 dollars new but ive seen them on here and ebay for like 150-200. i have the 78mm and will be doing this when i add heads/cam. u also need the 90mm Tb to maximize power gains.

 

anyone know if the rest of the intake is identical tho other than the top shells?

 

You only need the upper shell to convert it, the lower shell is the same. You should also get a new gasket that seals them together as well.