Why bother creating FITs Al? Unless you've done darks, flats, bias etc and have accurate calibration and a reference image for correction, you're not going to be submitting to a science archive!
Just use BASS Project software which will graph pretty much any image file, including jpegs. You can work your way into the serious stuff later if you develop sufficient interest. The software is available in this group:
https://uk.groups.yahoo.com/neo/groups/astrobodger/info
Attached are a few I graphed from your montage, calibration approximate based on the HB line in your EC spectrum. I feel your spectra are a bit overexposed or too much contrast or something in processing.
And you should run the spectra to the right, not left.
Keep it up!
Cheers -