FOV Type is only related to what the game uses to calculate the actual FOV, it's not relevant to the conversion or your aspect ratio. The only important metric for the conversion is the actual HFOV or VFOV. As CSGO is locked to 90 Hdeg 4:3 (unless you have cheats enabled), you don't need to change this based on your aspect ratio.
If you want to convert from 1280x960 stretched to 2560x1440 the only choices you have to make is if you want to match the horizontal or vertical sensitivity, and how you want to match them.
For vertical there's no difference, since only the horizontal axis is stretched. So the vertical sensitivity is identical in both cases. For the horizontal you have to decide if you want to match it based on 360 distance (which would be the same), monitor distance vertical 0%, 133.3333% or monitor distance horizontal 100% etc. Which one feels best is down to what you prefer.