Delta 50-580 Type and 7" inlet diameter

[transplant_wi]

I understand that there are three types of the Delta 50-850. Mine has a 7" inlet when you remove wye with the two 4" inlets. Does this identify it as a particular 'type', and does anybody know if I could buy a different inlet plate with a 6" aperture instead? Is there a way to identify the type, e.g. by serial number?

I could just fabricate an inlet cover from MDF and a 6" metal duct splice, but I wondered whether the slight conical shape of the cover is important for optimal airflow.

One parting thought is that the inlet's lip curves inwards to facilitate mounting the wye, so I might dremel-off the lip to remove a source of turbulence. Does this make sense?