So unless theres some way to convert with the bits themselves it sounds like I need a seperate PC program to work with the 64 bit floats. So after I do message order checking I'll have to pass the full message array to a PC to be changed to RTCM standard format or start with a different imbedded processor.
I was looking at the spec sheet for a AVR32UC which you can get with an FPU built in, but it looks like it is set up for 32 bit acess also.