Georeference a raster, CRS unknown but latitude and longitude grid is there

Question

So, the situation involves :

• QGIS 2.18 and georerencer plugin;
• Downloaded TIF of an old canadian topographical map from Quebec National Library and Archives (permalink here) and as you can see, this nice map included a grid with latitude and longitude.
• The project I'm working on includes vector data using NAD83 MTM7.

Now, the problem :

• Right after selecting file in the georeferencer plugin, I have to enter what i think is the CRS of the original map, which I don't have a lot of info on (It says "polyconic projection" only).
• I was under the impression that "real" lat/long would be more than enough to georeference it precisely, but it appears that a entering a CRS is essential!

EDIT : Here's a screen capture of the menu prompted when adding the raster. It's in french, but in the first field, it translates to "select the CSR for the raster layer"

Now, the questions are :

1. Is there any way I can avoid specifying a CRS when using georeferencer and just enter the lat long of the grid corner? Something that would tell QGIS "Look, forget the CRS, I have latitude and longitude and that all you need!"
2. If answer to question 1 is "No", what should I do?

Answer 1

If you load a new raster (at least in QGIS 2.18), you may get prompted for a CRS. I never understood why. It might be a global setting for adding new layers, but it has no effect at this stage.

In the settings of the georeferencer (the yellow gear-wheel icon), you have to specify the target CRS, and an output filename. If you want to enter degrees, I suggest to take NAD27, not WGS84 or NAD83. Keep in mind it is an old map!

You can not run the Georeferencer without specifying any CRS, because the output Geotif needs one. Remember that latlon degree CRS are also valid CRS!

In fact, you have a whole bunch of geographical CRS, all using latlon degrees, but on different ellipsoids, and some with a center meridian different from Greenwich.

Since the map is older than NAD27, you can not be sure what datum is actually used. So you might be better off by georeferencing against recognizable points (like street or railway crossings) compared to a current basemap like Openstreetmap or Google imagery via the QMS plugin. In that case, I suggest to set project and georeferencer CRS to EPSG:3857.