MP plenums compatible with late model blowers or MPIIs are no longer being produced by Magnum Powers. Victor may or may not have one...doubt it. MPII blowers are no longer being produced, the casting pattern for the rear plate was revised to the MPIII design. MPIIs and MPIIIs can only be made from an early model case. The MPIII can only be used with the matching MPIII plenum.
MPI is a ported late model blower that should provide similar performance to that of a Magnason S port. Unfortunatly, if you don't have an MP plenum for it, it's not going to flow very much air. The MPII & MPIII design positions the inlet higher than is possible by porting the late model case. According to Magnum Powers this higher inlet position is more efficent and flows more air.
So your choices are. Port your late model blower into an MPI if you can find an MP plenum (stock plunum sucks and even when ported will only have a 68mm opening) or have your early model case reworked into an MPIII which includes a matching high flow MP plenum. The latter will provide the best performance, but will also cost the most. There are also a few people who are selling their used MPIIs and matching MP plenums...just watch the classifieds because they sell quick.
David