Installing C-90 in a 415-C

My A&P told me that modifying my old C-75 to
a C-90 during the overhaul is not a problem but
the modification appears to disqualify it as a C
model even though the gross weight will not increase
above 1320 lbs. I read that this has been discussed
in the past but can find no reference or answer.
Can I install a C-90 and keep it a 415-C?