the inlet on the MPI is bigger than 40mm, so fitting a 40mm on an MPI seems daft to me.
see this thread;
https://www.fiatforum.com/cinq-sei-technical/129906-sei-mpi-throttle-body-enlargment-guide-idea.html
my bad, i thought the inlet on the plenum on the MPI was 43mm but it is only 39mm.
FYI, the inlet on the P75 plenum (without spacer) is 48mm. So if it were me i'd bin the Sei MPI manifold and fuel rail, and fit the P75 kit with a 16v TB on it.