Fortran Fits writer/reader
It looks like CFITSIO has an interface for F77. 

I'm always thinking about programmer time vs. computation time in research settings. If your data is reasonably small and simple in structure, I'd be very tempted to have Fortran write to a text file, and then use something like astropy to turn the text file into a FITS file. It's slow and icky, but it takes fifteen minutes of thinking if you've used astropy before.