Camera hole depends on what you want.
The small hole behind the pilots seat is easy. Seat rails are 10" apart so opening in the floor can be about 9". A bit wider on the outer skin. Floor depth at that point is 4.5”. Requires a doubler on the outer skin. It is in the earlier Cessna parts manuals so probably little paperwork required. Out here (in South Africa) anyway.
The big hole on the other side is about 19” diameter. That is a different matter altogether. It would be more economic to buy a complete 337 with it already fitted than to try to do a retrofit. Don’t even think about it.
Alternatively you could go the pop out baggage door route. One offering at
http://www.aerotopol.com/html/aeroshift.html There are STC’s available for the 172, 206 and 210. Not sure about the 337 but it couldn’t be very difficult thing to do.