Is it safe to convert a candelabra base in my ceiling fan to a medium base for use with larger CFL's?
You will be OK as far as not overloading electrically. You need to adhere to the most restrictive labeling, the 40wx3. Since you are using 27wx3, you are OK. The 75w limit is for the socket base only. It can handle 75w, but the rest of the fixture cannot. Be aware that 81 watts of fluorescent light is WAY more than the 120 watts of incandescent. If 120 watts was adequate, this could be overkill. There are various CFLs available in a range of light output, so you should be able to find an appropriate output.

Another consideration is weight. Some CFLs are rather heavy, and that weight cantilevered out by the adapter adds significant leverage on a socket only intended to support a small candelabra lamp. Multiplied by the fan vibrations, this could be an issue. Carefully evaluate how sturdy the sockets are mounted before proceeding with this change over.

And of course there is no way the bowl will fit over this sort of configuration. You will have the joy of everyone being able to see your handiwork :)